I agree with Molly above but have a lot to add. PGP (or GPG if you want something freeware) is very easy to use, and works with many standalone mail clients. That said, it won't work with email that you use in-browser (as far as I know) and both people need to have the same (or at least interworking) package installed.
This is not difficult, but convincing others to install it and use it can be hard. I know I tried a while back, and nobody would follow along.