Here's a ranked listing of Fonts.com Web Fonts' top 100 most used Web fonts for May 2012:

Neue Helvetica

Trade Gothic

Futura

DIN Next

Frutiger

Gill Sans

Avenir

Helvetica

Univers

ITC Avant Garde Gothic

Neue Frutiger

Futura T

Slate

Trade Gothic Next

New Century Schoolbook

PMN Caecilia

Avenir Next

Neo Sans

DIN 1451

Linotype Didot

ITC Lubalin Graph

Linotype Univers

Frutiger Next

Arial

Garamond 3

VAG Rounded

Neue Helvetica Arabic

Harmonia Sans

Times

Monotype News Gothic

Yakout

Bauer Bodoni

Soho

Heisei Kaku Gothic

Laurentian

Soho Gothic

Gothic

Rockwell

Eurostile LT

Egyptian Slate

Cachet

Glypha

ITC Franklin Gothic

Memo

Century Gothic

Monotype Grotesque

Plantin

Akko

Camphor

Neue Helvetica eText

Sackers Gothic

Plate Gothic MT

News Gothic

ITC Officina Sans

Clarendon

ITC Stone Informal

Monotype Modern

Aachen

Ocean Sans

Amasis

FB Han Gothic

Bell

ITC American Typewriter

Azbuka

Consolas

Andale Mono

MYuppy

Droid Sans Mono

Frutiger Serif

Eurostile Next

Linotype Feltpen

Walbaum

Optima

ITC Garamond

Mitra

Wiesbaden Swing

Baskerville

Neue Haas Grotesk

Adelle

ITC Conduit

Bembo

Monotype Garamond

Impact

ITC Kabel

ITC Franklin

Neuzeit Office

Calibri

Rotis Sans Serif

Gazette

ITC Legacy Serif

Klint

ITC Officina Serif

Abadi

ITC Caslon No. 224

Loft

Museo

Neo Tech

Calvert

P22 Underground

Bodoni LT