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.