The free icon sets are available in many different formats, with SVGs, as mentioned above, heavily represented, but there are also many sets in PSD, AI, EPS and Sketch formats. All of the sets are free to download, and most will allow you to use them in both your personal and commercial projects, but please do check the license before you do use them.
Top 50 Free Icon Sets 2014
Linea Iconset (730+ Icons, SVG, PNG & Webfont)
3px Icons Vector Set (80 Icons, AI & PSD)
3px Icons Vector Set Download Page →
Themify Icons (320+ Icons, SVG & Webfont)
GitHub’s Octicons (178 Icons, Webfont)
GitHub’s Octicons Download Page →
Simple Line Icons (100+ Icons, AI, EPS, SVG & PSD)
Simple Line Icons Download Page →
Freecons (155 Icons, AI, EPS & PSD)
iOS7 Vector Icons (100 Icons, Photoshop Custom Shape, PNG & Webfont)
iOS7 Vector Icons Download Page →
Odincons 1.0 (100 Icons, PSD)
Genericons (120 Icons, Webfont)
Elegant Themes Line-Style Icons (100 Icons, AI, SVG, PNG & Webfont)
Elegant Themes Line-Style Icons Download Page →
Flat Icon Set (60 Icons, PNG, SVG, EPS, PSD & AI)
Stroke Gap Icons Set Vol.2 (50 Icons, PSD & AI)
Stroke Gap Icons Set Vol.2 Download Page →
Stripes & Co Line-Styled Icon Set (65 Icons, AI & EPS)
Stripes & Co Line-Styled Icon Set Download Page →
Open Iconic (220 Icons, SVG & Webfont)
Stroke 7 Icon Font (170 Icons, Webfont)
Stroke 7 Icon Font Download Page →
Meroo Flat-Styled Icon Set (110 Icons, PSD)
Meroo Flat-Styled Icon Set Download Page →
Dashicons WordPress Icon Set (190 Icons, Webfont)
Dashicons WordPress Icon Set Download Page →
Stroke Icons (80 Icons, PSD, AI & Webfont)
Metrize Metro-Style Icons (300 Icons, PSD, SVG, EPS, AI, PDF & Webfont)
Metrize Metro-Style Icons Download Page →
Outlined Icons (150 Icons, PSD, AI, SVG & Webfont)
Outlined Icons Download Page →
Feather Icon Set (130 Icons, PSD CSH, SVG & Webfont)
Feather Icon Set Download Page →
Iconia (Regular, Circle & Cut) (113 Icons, Webfont)
Ballicons 2 Icon Set (70 Icons, PNG, PSD, SVG)
Ballicons 2 Icon Set Download Page →
Summer And Essentials Icon Set (50 Icons, EPS, AI, PNG)
Summer And Essentials Download Page →
Blackberry Icons (20 Icons, PSD)
Blackberry Icons Download Page →
Mintie Flat Icon Set (110 Icons, PSD & PNG)
Mintie Flat Icon Set Download Page →
Fanaticons (64 Icons, AI, PNG & SVG)
Jellycons iOS 8 App Icon Set (PNG & Sketch)
Jellycons iOS 8 App Icon Set Download Page →
Musicons (72 Icons, PSD)
Gallericons (30 Icons, Sketch & PNG)
The Web and Mobile Line IconSet (200 Icons, AI)
The Web and Mobile Line IconSet Download Page →
Bubbles iOS 7 Iconset (48 Icons, AI, EPS, PNG, PSD & SVG)
Bubbles iOS 7 Iconset Download Page →
Wireframe Icons (230 Icons, PSD)
Wireframe Icons Download Page →
Icon-Works (134 Icons, Icon Font)
Webicons (100 Icons, AI & EPS)
Line Icons (60 Icons, PSD, AI & EPS)
12k Line Icon Set (12,000 Icons, PSD)
12k Line Icon Set Download Page →
The Web Interface Icon Set (370 Icons, SVG & AI)
The Web Interface Icon Set Download Page →
Devicons – An Iconic Font For Developers (123 Icons, Icon Font, SVG & PNG)
Solid Icons (210 Icons, PSD, PNG, SVG & Icon Font)
Compacticons (180 Icons, PSD)
UI/UX Icons (40 Icons, PSD, AI & SVG)
Touch Gestures Icons (100 Icons, AI, EPS, PSD, PNG & SVG)
Touch Gestures Icons Download Page →
Helium Icon Set (100 Icons, AI, EPS, SVG & Icon Font)
Helium Icon Set Download Page →
Material Design Icons from Google (750 Icons, PNG)
Material Design Icons Download Page →
UNigrid Vector Icons (100 Icons, AI)
Touch Icon Set (340 Icons, AI, PSD & SVG)
Touch Icon Set Download Page →