UseModWiki is wiki software written by Clifford Adams in the Perl programming language. Until early 2002 it was used to run all versions of Wikipedia. Currently it is still used for a number of language versions, including Finnish, Italian, Portuguese, Norwegian and Lower Saxon. They are planned to be converted to MediaWiki, see m:Wikipedia software upgrade status.
Also many other wikis use UseModWiki, including MeatballWiki and the ApacheWiki. Usemod is probably the most popular wiki software (in terms of number of wikis, not in terms of total number of edits), despite the fact that it has few features, because it doesn't take as much computing power as other wiki software implementations such as TWiki or Wikipedia's MediaWiki software.