Icon Fonts (or pictograms) are available for a very long time but the ones that are specifically designed for web apps are appearing lately.
They are so flexible and fun to use considering each icon can be resized, colored, rotated and styled very easily. No need to hack them in our image editing apps but just CSS is enough.
And, making buttons, menus, lists, etc. with icons becomes painless as we deal with a single element: "the text" rather than "the text and the images/icons".
Previously, pictograms for common-use were shared (Designing With Fonts: 30 Useful & Free Dingbats). But, this time, we come up with the ones that are perfect for websites and web apps, the ones that may replace your icon sets. And, they are free.
Entypo (100+ items)
Entypo is a high-quality icon font with 100+ items and available in OpenType font, vector EPS and web font.
Font Awesome (150+ items)
Font Awesome is designed to be used with Twitter Bootstrap but also can fit into any other web project.
There are 150+ items, comes with various font formats and includes CSS + LESS styles.
Modern Pictograms (90+ items)
A set of 90+ slick pictograms which can be used in any size (but work much better in >24px).