roWiki
A neat little wiki run by a ~150 line PHP script. Sadly, the creator seems to have taken down the official site, so a mirror is presented here. Because the thing is pretty much impossible to find nowadays.
FlatBlog
A minimilist blogging system that uses text files to store posts. Handy if you don't have a database, or you just want a simple PHP blogging application to study. Again, the creator's site has vanished.