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

Neue Helvetica

Trade Gothic

Futura

Frutiger

Gill Sans

Avenir

Helvetica

DIN Next

Univers

ITC Avant Garde Gothic

Neue Frutiger

Avenir Next

New Century Schoolbook

PMN Caecilia

Trade Gothic Next

Linotype Univers

Neo Sans

Times

Neue Helvetica Arabic

DIN 1451

Linotype Didot

Arial

Frutiger Next

Harmonia Sans

Slate

VAG Rounded

Garamond 3

Memo

Yakout

Monotype News Gothic

Soho

Bauer Bodoni

Frutiger Serif

Sackers Gothic

Laurentian

ITC Officina Sans

Rockwell

Gothic

Futura T

Heisei Kaku Gothic

Cachet

Eurostile LT

Glypha

Soho Gothic

ITC Franklin Gothic

Akko

Century Gothic

Egyptian Slate

ITC Lubalin Graph

Plate Gothic MT

Monotype Grotesque

ITC Legacy Serif

Monotype Garamond

Calibri

Optima

Museo

Plantin

Neue Helvetica eText

Adelle

Neue Haas Grotesk

ITC Officina Serif

Clarendon

Aachen

Ocean Sans

News Gothic

ITC Stone Informal

Amasis

ITC Garamond

Consolas

Andale Mono

Droid Sans Mono

Bell

Monotype Modern

Klint

Azbuka

ITC Conduit

ITC American Typewriter

ITC Franklin

Wiesbaden Swing

MYuppy

Baskerville

Impact

Eurostile Next

Georgia

Linotype Feltpen

Camphor

Mitra

Gazette

P22 Underground

ITC Caslon No. 224

Calvert

Bembo

Rotis Sans Serif

Neuzeit Office

Memphis

Serifa

Agilita

Abadi

Janson Text

Loft