Attention computer animators -- if you've ever felt limited by working in three dimensions with tools like Blender, check out Pencil, an open source, cross-platform animation app that lets you create in glorious 2-D. Pencil mimics hand-drawn animation techniques, but it's easy to use and produces high-quality output.
You can download source code packages as well as pre-built binaries for Ubuntu, Arch Linux, Windows, and Mac OS X. The latest release is 0.4.4b for Macs and Windows boxes, and 0.4.3b for Linux. The differences between the two versions are minor, but include a change to the file format, so if you run Pencil on multiple platforms, you may want to stick with 0.4.3b until the Linux build is updated.
Creating an animation in Pencil is easy. A timeline at the bottom of the screen lets you step through individual frames. You can incorporate multiple layers, each of which can be of either bitmap or vector format. Note that a vector layer in this context means that the layer is drawn with vector curves -- it does not incorporate vector animation as in Adobe Flash or the free Synfig animation studio. In vector animation, you create motion by moving objects between keyframes, and the application calculates and draws all of the intermediate frames.