Ian Haigh announced Ease and Wizz, a donationware set of scripts for After Effects that gives you more ways to easily interpolate between values. The easing types include Expo, Circ, Quint, Quart, Quad, Sine, Back, Bounce, and Elastic; some are shown in a short video example and in an interactive visualizer. Ian explains:
"The obvious use is in motion, but they can be used on animated properties of any kind. They’re controlled with a After Effects-ish palette that can be docked, so it’s very easy to use. A while back, Flash guru Robert Penner created a suite of extremely useful easing equations that have been used to build thousands of websites worldwide. I've adapted these equations to work as expressions in After Effects."
Update: Alan Shisko adds, "There are two drawbacks to the script, both with workarounds. One, it only works on the first two KF's in an a given parameter. The workaround is to apply the script on multiple 'transform' EFFECTS instead of on to multiple keyframes on the 'transform' PARAMETER. And two, you can't view the resulting velocities in the Graph Editor (should you so choose) unless you select the KF's and go to Animation-> Keyframe Assistant-> Convert Expression to Keyframes. But I would surely recommend that you give it a try, I think you'll like it."