In addition to MediaWiki that was mentioned by Kenny, you might also look at MoinMoin.
Choosing between MediaWiki and MoinMoin can be a bit tough. Here are some points to consider:
Fairly easy to set up.
Made soley for wikipedia. Thus it can be a bit of a pain to customize how you like it.
Huge amount of plugins and third party modules available.
Can be a pain to install.
There are a huge amount of other wikis available, but those are the main two I would consider.