user323: How often do forks occur in the block chain? I'm interested in the small, one block forks that happen when two miners create a block at the similar time.

user3578: Both of the links in the previous two answers aren't working.
Historic data (both, until March 2014) can be viewed on the internet archive.
The last snapshot from blockexplorer.com: https://web.archive.org/web/20130313092352/http://blockexplorer.com/q/reorglog 
The last snapshot from blockchain.info can be found in a similar way (stackexchange doesn't allow me to post more than 2 links, since I don't have enough reputation...). 
An updated information, that unfortunately contain few details, is available in Organ Ofcorti's blog.
See, for example, http://organofcorti.blogspot.com/2014/05/may-11th-2014-weekly-hashrate.html , Table 2.
The information is presented per week, in an image (not as text - which makes it a bit harder to analyze).  

user1370: Update for everyone who stumbles up on this question. There is a paper that shows why it is hard to figure out the actual fork rate.

it's from this paper "Echoes of the Past:
Recovering Blockchain Metrics From Merged Mining" 
https://eprint.iacr.org/2018/1134.pdf

user-1: I don't think the blockexplorer reorglog catches all blocks. http://blockchain.info/orphaned-blocks shows  1-2 detached blocks per day.

user132: Blocks are orphaned once in a while, roughly every few hundred blocks:
http://blockchain.info/orphaned-blocks
There used to be a blockexplorer-provided list of chain reorganizations which can occur upon such events (might become available again someday):
http://blockexplorer.com/q/reorglog
These chain reorgs used to happen a few times per month.

323: <p>How often do forks occur in the block chain? I'm interested in the small, one block forks that happen when two miners create a block at the similar time.</p>
 3578: <p>Both of the links in the previous two answers aren't working.
Historic data (both, until March 2014) can be viewed on the internet archive.
The last snapshot from blockexplorer.com: <a href="https://web.archive.org/web/20130313092352/http://blockexplorer.com/q/reorglog" rel="nofollow">https://web.archive.org/web/20130313092352/http://blockexplorer.com/q/reorglog</a> </p>

<p>The last snapshot from blockchain.info can be found in a similar way (stackexchange doesn't allow me to post more than 2 links, since I don't have enough reputation...). </p>

<p>An updated information, that unfortunately contain few details, is available in Organ Ofcorti's blog.<br>
See, for example, <a href="http://organofcorti.blogspot.com/2014/05/may-11th-2014-weekly-hashrate.html" rel="nofollow">http://organofcorti.blogspot.com/2014/05/may-11th-2014-weekly-hashrate.html</a> , Table 2.
The information is presented per week, in an image (not as text - which makes it a bit harder to analyze).  </p>
 1370: <p>Update for everyone who stumbles up on this question. There is a paper that shows why it is hard to figure out the actual fork rate.</p>

<p><a href="https://i.stack.imgur.com/FwEpT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FwEpT.png" alt="xxx"></a></p>

<p>it's from this paper "Echoes of the Past:
Recovering Blockchain Metrics From Merged Mining" </p>

<p><a href="https://eprint.iacr.org/2018/1134.pdf" rel="nofollow noreferrer">https://eprint.iacr.org/2018/1134.pdf</a></p>
 -1: <p>I don't think the blockexplorer reorglog catches all blocks. <a href="http://blockchain.info/orphaned-blocks">http://blockchain.info/orphaned-blocks</a> shows  1-2 detached blocks per day.</p>
 132: <p>Blocks are orphaned once in a while, roughly every few hundred blocks:</p>

<p><a href="http://blockchain.info/orphaned-blocks" rel="nofollow">http://blockchain.info/orphaned-blocks</a></p>

<p>There used to be a <a href="http://blockexplorer.com/" rel="nofollow">blockexplorer</a>-provided list of chain reorganizations which can occur upon such events (might become available again someday):</p>

<p><a href="http://blockexplorer.com/q/reorglog" rel="nofollow">http://blockexplorer.com/q/reorglog</a></p>

<p>These chain reorgs used to happen a few times per month.</p>
