Hmmm... first post because I forgot what username I had previously LOL.
I am one of the team with Mambo Guru, whose wiki was mentioned by Chuck. When I was looking at the various wiki options for our online manual, I went through all the more popular wikis and checked them out. All of them have pros and cons, but for our purposes, mediawiki really stood out from the rest as the most likely to do what I wanted. The wiki had to be solid, with sound development to limit the risk of hacking, it had to easily enable only registered users while preventing unregistered folk from editing, it had to have good search functions, and had to be able to handle a lot of visitors and editors without excessive server loads (we plan for growth
Security was one of the most important considerations as we anticipated problems.
I also wanted some decent control on the administrative side as I have seen too many wikis get cluttered with author profiles that essentially just promoted the authors and their sites, and have also seen too much good work get defaced by unscrupulous people. DocuWiki was quite good, but not as fully featured as MediaWiki - the others were quickly tested and discarded for various reasons.
Since setting up our MediaWiki, I have been very happy with the choice I made. We are using it purely for documentation but it would lend itself to managing snippets with no problems.
The downside, which applies to all wikis out there, is that although URLs can be rewritten to be very user and search engine friendly, search engines pick up a lot of unneccessary content - things like the author page, edit pages, etc and this can result in search engine penalties.
If you do decided to go with MediaWiki, have a search for the skins done by Robert Castley. I used one as a base for our wiki and it sure saved a lot of time in making the wiki look less "wiki like". Mine is fairly heavily modified, or I would offer it. Robert’s would be easier to work with to make your wiki more integrated with the site look and feel.
Hope this helps.