I would advise either
- Foswiki ( http://foswiki.org ), (forked by the whole developer community of TWiki to avoid trademark threats), for a feature-rich and fully open programmer's wiki. Drop on #foswiki on irc.freenode.net to chat with the community.
- Mindtouch's Deki Wiki ( http://www.mindtouch.com/ ) clearly the most user-friendly advanced and innovative wiki out there, a modern commercial + open source offering. Great integration with Office docs.
I would avoid Confluence. Confluence made a design choice (forbidding mixing html in pages with Wiki syntax) that proves deadly to any attempt at wysiwyg, as it uses a standard HTML editor for WYSIWYG, and this converts it on save in a very limited subset of it, yielding frustrating surprises for the users (foswiki for instance keeps as html the parts the wiki syntax do not handle like bullet lists in table cells). Confluence have many great sides, notably its integration with atlassian great tools as their JIRA bugtracker, (we use it at work for this with good results) but do not plan to customize it.
There are many good choices on hosted wikis too (Google sites, based on the awesome jotspot engine is one).
Never use Sharepoint of course. Its wiki capabilities are a IE-only joke, and Sharepoint whole architecture is braindead (storing all data - even huge docs - in a non-distributed database goes against Microsoft own recommendations). If you want a DMS with good Office integration, have a look at KT (Knowledge Tree) instead. http://www.knowledgetree.com/ . For political reasons we were forced to use Sharepoint at work but we limited it to basic document managing (never use the MOSS higher layer, as it breaks compatibility between versions) and integrated a foswiki frontend to it (dumped document list & metadata in xml and provided navigation in foswiki, and search with a google box)
But my real advice would be to ... wait for Google wave, that promises to revolutionize the wiki concepts.
Disclaimer: I am part of the foswiki community.