user58: The block chain file is getting rather large, ~500MB.  Is there a way to trim it or clean up, or maybe configure it to be limited in the future?  I am using the stock Bitcoin client on Ubuntu.

user4774: NOTE: Do not use this solution.  It is no longer relevant for current clients. See other answers for suggestions on how to use incomplete blockchain representations. 
The current version of the client doesn't offer much in the way of optimizing space, and does some caching that may cause bloat, but here are some ideas for manual ways that could help.

Copy the wallet.dat file out of the data directory, then remove the data directory.  Restart the client have it build the data directory again and download the block chain. Stop the client replace the wallet.dat file with your backup. Restart the client with the -rescan option.
Backup your entire data directory and remove it. Start the client again creating a new data directory, and block chain download. Copy an address from the client. Close the client, backup the new data directory as a different backup.  Restore the old directory.  Start the client and transfer all of your coins to the new address.  You can then restore the number 2 backup of the data directory, and discard the old directory. In this scenario you do not need to restart the client with the -rescan option.

However, be very careful with these steps, and keep all the backups until you are sure you are successful.  And remember that if you are expecting payments on the old wallet.dat file then you shouldn't use option two unless you can easily update the payment address to your new wallet.

user85: The short answer is: No.
The Bitcoin client requires the entire block chain in order to validate transactions, and the block chain is currently around 500MB. It will continue to get larger and larger over time.

user323: If you want to use the standard client, currently there is no way to prune the blockchain data. However, you can try using a lightweight client (like MultiBit or Electrum), which offers similar security to the standard client (wallet.dat is stored on your computer), or an eWallet (like StrongCoin, or even using MtGox if you don't need too much functionality), but then you have to trust that service.
The first two would reduce the disk space on your machine, but you'd still need to synchronize with the network; the second two would make you store nothing on computer and you would not need to synchronize.
There is work being done on a "Stratum client", which might offer the speed and minimal disk requirements of an eWallet with security of a lightweight client.
You can read more about various levels of security provided by different types of clients here.

user22: For portable devices there are variants of the Bitcoin client often dubbed "selfish clients" in that they do not download any portion of the block chain other than what pertains to their own specific addresses and transactions. To my knowledge, none of these variants has been fully developed into a desktop client, but it is likely that such projects are underway. Unless your system is literally lacking the hardware to accommodate these files, however, it is recommended that you use a non-selfish client version. The lightweight versions are called "selfish" for a reason - by only holding block chain data which pertains to your own transactions, you cannot pass anything but that data to other nodes and as such your client is of minimal (if any) use to the network.

58: <p>The block chain file is getting rather large, ~500MB.  Is there a way to trim it or clean up, or maybe configure it to be limited in the future?  I am using the stock Bitcoin client on Ubuntu.</p>
 4774: <p><strong>NOTE: Do not use this solution.  It is no longer relevant for current clients. See other answers for suggestions on how to use incomplete blockchain representations.</strong> </p>

<p>The current version of the client doesn't offer much in the way of optimizing space, and does some caching that may cause bloat, but here are some ideas for manual ways that could help.</p>

<ol>
<li>Copy the wallet.dat file out of the data directory, then remove the data directory.  Restart the client have it build the data directory again and download the block chain. Stop the client replace the wallet.dat file with your backup. Restart the client with the -rescan option.</li>
<li>Backup your entire data directory and remove it. Start the client again creating a new data directory, and block chain download. Copy an address from the client. Close the client, backup the new data directory as a different backup.  Restore the old directory.  Start the client and transfer all of your coins to the new address.  You can then restore the number 2 backup of the data directory, and discard the old directory. In this scenario you do not need to restart the client with the -rescan option.</li>
</ol>

<p>However, be very careful with these steps, and keep all the backups until you are sure you are successful.  And remember that if you are expecting payments on the old wallet.dat file then you shouldn't use option two unless you can easily update the payment address to your new wallet.</p>
 85: <p>The short answer is: No.</p>

<p>The Bitcoin client requires the entire block chain in order to validate transactions, and the block chain is currently around 500MB. It will continue to get larger and larger over time.</p>
 323: <p>If you want to use the standard client, currently there is no way to prune the blockchain data. However, you can try using a lightweight client (like <a href="http://multibit.org/" rel="nofollow noreferrer">MultiBit</a> or <a href="http://ecdsa.org/electrum/" rel="nofollow noreferrer">Electrum</a>), which offers similar security to the standard client (wallet.dat is stored on your computer), or an eWallet (like <a href="https://strongcoin.com/" rel="nofollow noreferrer">StrongCoin</a>, or even using <a href="https://mtgox.com/" rel="nofollow noreferrer">MtGox</a> if you don't need too much functionality), but then you have to trust that service.</p>

<p>The first two would reduce the disk space on your machine, but you'd still need to synchronize with the network; the second two would make you store nothing on computer and you would not need to synchronize.</p>

<p>There is work being done on a <a href="https://bitcointalk.org/index.php?topic=55842.180" rel="nofollow noreferrer">"Stratum client"</a>, which might offer the speed and minimal disk requirements of an eWallet with security of a lightweight client.</p>

<p>You can read more about various levels of security provided by different types of clients <a href="https://bitcoin.stackexchange.com/questions/2613/how-secure-are-various-models-of-bitcoin-clients">here</a>.</p>
 22: <p>For portable devices there are variants of the Bitcoin client often dubbed "selfish clients" in that they do not download any portion of the block chain other than what pertains to their own specific addresses and transactions. To my knowledge, none of these variants has been fully developed into a desktop client, but it is likely that such projects are underway. Unless your system is literally lacking the hardware to accommodate these files, however, it is recommended that you use a non-selfish client version. The lightweight versions are called "selfish" for a reason - by only holding block chain data which pertains to your own transactions, you cannot pass anything but that data to other nodes and as such your client is of minimal (if any) use to the network.</p>
