Valve announced today that its new Steam screenshots feature is now available. A quick hotkey press (F12 by default) takes a screenshot of (almost) any game run through the Steam client. You can then publish the screens to your Steam Community profile, Facebook, Twitter, or Reddit.
Restarting Steam will bring the latest update, which enables the feature. The shots you upload are stored on Steam Cloud, which has 1GB of storage space, so that should be plenty.
The screenshots feature was previously only available to beta users, in an update that added a host of other features. The full update includes a Library search feature, voice improvements, and general bug fixes, among other tweaks.