it is by far the most used with Scholarpedia a distant 2nd “with only… 5% of the number of papers referencing Wikipedia.”

All other free online encyclopedias tallied less than 50 papers referencing them in 2011.

Citizendium: “an English-language free encyclopaedia project launched by Wikipedia’s co-founder.”

Knol: “Knol is a Google project including user-written articles on a range of topics.”

PlanetMath: “a collaborative encyclopaedia focussing on mathematics.”

Scholarpedia: “peer-reviewed open-access encyclopedia, where knowledge is curated by communities of experts.”

Wikibooks: “a free library of educational textbooks that anyone can edit.”

Wikipedia: “a free, collaborative, multilingual Internet encyclopedia.”

Wikisource: “Wikisource is an online library of free content publications, collected and maintained by the Wikisource community.”

Data from Scopus.  Reference from: Research Trends (bibliometrics newsletter) Issue 27, March 2012.  “The influence of free encyclopedias on science”, by Sarah Huggett.