user238: Every time I turn on the Bitcoin client it takes days to catch up with the block chain.
Are there alternatives to getting it trickling through the peer-to-peer network? Can I just download it somewhere? Wouldn't this be ideal for distribution via BitTorrent? 
If so, how do I import it into the Bitcoin client?
My client would still be validating this data (can it?), and it is all signed and proof-of-worked, so there is no real security problem here, is there?

user2769: (edit: removed old and stale magnet)
now use these, as advised in top answers:
https://bitcointalk.org/index.php?topic=145386.0
https://bitcoin.org/bin/blockchain/
http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/bootstrap.dat.torrent/download
http://armoryguide.com/bootstrapping-the-blockchain/

user11946: Bootstrapping will be deprecated as of the 0.10 release in a few weeks. The torrent file mentioned in other answers will never be updated again because it is faster to have Bitcoin Core sync over the network. 
So no need to download the Blockchain seperately anymore!
You can find the release notes here https://github.com/bitcoin/bitcoin/blob/0.10/doc/release-notes.md

user-1: http://en.blockchaindownload.nl/ is a good way to download the full blockchains of bitcoin and litecoin. The site is new so i expect more improvements in the future.
They offer free torrent downloads uploaded 24/7 by 3 servers so bandwith is pretty good and the last upload at the time of writing this was a nicely compressed rar file!

user3547: "Bitcoin-Qt/bitcoind version 0.7.1 and later supports a special import feature:  If the file "bootstrap.dat" is found in the bitcoin data directory, it will validate and import all blockchain data found in that file."
The official torrent can be found at: http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/
Magnetic link: magnet:?xt=urn:btih:6fe493ba606847eac163baf35aae9db319735482&dn=bootstrap.dat&tr=udp://tracker.openbittorrent.com:80&tr=udp://tracker.publicbt.com:80&tr=udp://tracker.ccc.de:80&tr=udp://tracker.istole.it:80
Thread: https://bitcointalk.org/index.php?topic=145386.0

user85: This was an issue with versions of the client prior to 0.3.24 -- once the vast majority of the network switches over, the block chain downloads should proceed as fast as your CPU and network connection can handle.
There are places you can download the block chain, but the client doesn't provide a way to accept the block chain in any form other than from the network. If you're starting from nothing, it will generally take about 10 hours to process the whole block chain. Most of it isn't transfer time, it's CPU time validating all the hashes and signatures.

user54: A good start of downloading block chain is here
http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/
update 22.02.2021: URL is not more Working

user108: Note by Murch: This download link is broken and it seems unlikely to me that anyone still provides this service. Network and synchronization code have been immensely improved over the years so that the initial block download via the network is usually acceptably fast.
You can download the blockchain nightlies handled by tcatm (one of the Bitcoin core developers and the operator of bitcoincharts.com). Unpack and copy&replace blkindex.dat, blk0001.dat files into C:\Users\"username"\AppData\Roaming\Bitcoin for Windows 7, and run bitcoind with the -rescan option. Then it will take only a couple minutes to download up to latest block. Be aware that manually copied blocks won't get verified.

238: <p>Every time I turn on the Bitcoin client it takes days to catch up with the block chain.</p>

<p>Are there alternatives to getting it trickling through the peer-to-peer network? Can I just download it somewhere? Wouldn't this be ideal for distribution via BitTorrent? </p>

<p>If so, how do I import it into the Bitcoin client?</p>

<p>My client would still be validating this data (can it?), and it is all signed and proof-of-worked, so there is no real security problem here, is there?</p>
 2769: <p>(edit: removed old and stale magnet)</p>

<p>now use these, as advised in top answers:</p>

<p><a href="https://bitcointalk.org/index.php?topic=145386.0" rel="nofollow">https://bitcointalk.org/index.php?topic=145386.0</a></p>

<p><a href="https://bitcoin.org/bin/blockchain/" rel="nofollow">https://bitcoin.org/bin/blockchain/</a></p>

<p><a href="http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/bootstrap.dat.torrent/download" rel="nofollow">http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/bootstrap.dat.torrent/download</a></p>

<p><a href="http://armoryguide.com/bootstrapping-the-blockchain/" rel="nofollow">http://armoryguide.com/bootstrapping-the-blockchain/</a></p>
 11946: <p>Bootstrapping will be deprecated as of the 0.10 release in a few weeks. The torrent file mentioned in other answers will never be updated again because it is faster to have Bitcoin Core sync over the network. </p>

<p>So no need to download the Blockchain seperately anymore!</p>

<p>You can find the release notes here <a href="https://github.com/bitcoin/bitcoin/blob/0.10/doc/release-notes.md" rel="nofollow">https://github.com/bitcoin/bitcoin/blob/0.10/doc/release-notes.md</a></p>
 -1: <p><a href="http://en.blockchaindownload.nl/" rel="nofollow">http://en.blockchaindownload.nl/</a> is a good way to download the full blockchains of bitcoin and litecoin. The site is new so i expect more improvements in the future.
They offer free torrent downloads uploaded 24/7 by 3 servers so bandwith is pretty good and the last upload at the time of writing this was a nicely compressed rar file!</p>
 3547: <p><em>"Bitcoin-Qt/bitcoind version 0.7.1 and later supports a special import feature:  If the file "bootstrap.dat" is found in the bitcoin data directory, it will validate and import all blockchain data found in that file."</em></p>

<p>The official torrent can be found at: <a href="http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/">http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/</a></p>

<p>Magnetic link: magnet:?xt=urn:btih:6fe493ba606847eac163baf35aae9db319735482&amp;dn=bootstrap.dat&amp;tr=udp://tracker.openbittorrent.com:80&amp;tr=udp://tracker.publicbt.com:80&amp;tr=udp://tracker.ccc.de:80&amp;tr=udp://tracker.istole.it:80</p>

<p>Thread: <a href="https://bitcointalk.org/index.php?topic=145386.0">https://bitcointalk.org/index.php?topic=145386.0</a></p>
 85: <p>This was an issue with versions of the client prior to 0.3.24 -- once the vast majority of the network switches over, the block chain downloads should proceed as fast as your CPU and network connection can handle.</p>

<p>There are places you can download the block chain, but the client doesn't provide a way to accept the block chain in any form other than from the network. If you're starting from nothing, it will generally take about 10 hours to process the whole block chain. Most of it isn't transfer time, it's CPU time validating all the hashes and signatures.</p>
 54: <p>A good start of downloading block chain is here</p>
<hr>http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/<hr>
<p><strong>update 22.02.2021: URL is not more Working</strong></p>
 108: <p><em>Note by Murch: This download link is broken and it seems unlikely to me that anyone still provides this service. Network and synchronization code have been immensely improved over the years so that the initial block download via the network is usually acceptably fast.</em></p>

<p>You can download the <a href="http://eu1.bitcoincharts.com/blockchain/" rel="nofollow noreferrer">blockchain nightlies</a> handled by tcatm (one of the Bitcoin core developers and the operator of bitcoincharts.com). Unpack and copy&amp;replace <code>blkindex.dat</code>, <code>blk0001.dat</code> files into <code>C:\Users\"username"\AppData\Roaming\Bitcoin</code> for Windows 7, and run <code>bitcoind</code> with the <a href="https://en.bitcoin.it/wiki/Running_Bitcoin" rel="nofollow noreferrer"><code>-rescan</code> option</a>. Then it will take only a couple minutes to download up to latest block. Be aware that manually copied blocks won't get verified.</p>
