Windows only: Screen capture tool Screenpresso takes screenshots of part of your screen, a specific window, or the whole screen with a shortcut key, then lets you edit, add effects, and even share with email or Twitter.
Once you've installed the utility, you can immediately begin taking a region capture of your screen with the Print Screen key — it's the same functionality you're used to from any number of great screen capture tools, but Screenpresso will guess the region you are trying to capture based on the position of your mouse, allowing you to capture just the content area of an application with a single click. You can still click and drag to select a specific region, but it's a useful feature that could save you some time.
Once you've taken your screenshot, it will be saved into the history window, where you can access all of the screenshots you've taken, and either choose to share them with email or Twitter, or open them up in the included editor application, which lets you add arrows, shadows, text, or crop and resize the images.
There's loads of other options to choose from, and while it's still not quite got the polish of non-free SnagIt, it's a very capable free alternative that's definitely worth a look. Screenpresso is a free download for Windows, though readers should note anonymous usage statistics can't be turned off in the free version.