How often forks occur? 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:

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.