UseModWikiwiki software written by Clifford Adams in the Perl programming language. Until early 2002 it was used to run all versions of Wikipedia. All languages have since been converted to MediaWiki software.
Also, many other wikis use UseModWiki, including MeatballWiki. 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.