Hey everyone! I just discovered a neat little tool called
Fulla that corrects lens distortion.
It also corrects chromatic aberration and vignetting, but I haven't tried those out. Here's an example...
Before:

After:

As you can see, it works pretty well. One caveat, though: it depends on
a database from 2006; so if you don't have an ancient camera it won't be in the database. However, there may be a way to determine the distortion coefficients of newer cameras and then use those coefficients with Fulla, I just haven't looked into it. Another alternative would be to use
PTLens, which comes with an up to date version of the database mentioned earlier; but at the time of this post, PTLens is $25, which seems a little pricey.