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.
Blocks are orphaned once in a while, roughly every few hundred blocks:
There used to be a blockexplorer-provided list of chain reorganizations which can occur upon such events (might become available again someday):
These chain reorgs used to happen a few times per month.
I don't think the blockexplorer reorglog catches all blocks. http://blockchain.info/orphaned-blocks shows 1-2 detached blocks per day.
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"
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).