For a good introduction on algorithms, see Introduction To Algorithms
Here are some of the pages on wiki that describe algorithms
Wiki-related algorithms
The following are really more like features than algorithms, but perhaps some of them use interesting algorithms to do the work.
Diff Algorithm -- This is how a function comparing two pages on wiki (or two files in general) could work. See Wagner Fisher Algorithm
Adding New Pages -- How to create a wiki page.
Miscellaneous algorithms
Perpetual Calendar Algorithm -- A few lines of algorithm and a few lines of code and you will have any month after 1752 appear on the screen!
Uncategorized
c2.com 
gives a page list.
See original on c2.com