CGI::Kwiki is a nice, simple, extensible Perl wiki, described in more detail by the author in perl.com .
Also very handy is VoodooPad, a notepad application for OS X that works like a wiki (with the advantage of being able to edit the page directly). V1.1 includes a script for importing a folder of text files (my previously preferred method of storing notes).
Kwiki includes handy plugins such as a Powerpoint-style slideshow, and has very sensible formatting rules. VoodooPad is particularly suited to scientific writing for this reason, as it automatically turns acronyms, such as CD4, IL-12 and TLR-4 into wiki links - UseMod doesn't do this, which is why I had to stop using it. Maybe there's a patch somewhere Update: see comments.