One to check out that has been used internally here is DekiWiki ("enterprise-friendly"). We've also began to use it on external collaborative projects. For dev you can't beat Trac right now but Redmine may just superceed Trac. Ones I tend to avoid are TWiki (hugely extensible, but therein lies its weakness) and horrors of all horrors - sharepoint. MediaWiki of "old" is great if you need something up quicky.