I've found that the best way to help an open source project is with documentation. Not necessarily user manuals, but just code commenting. There's plenty of code that has zero explanation about what a particular function does, or how it works. If you've found a project that fits this, then start working on documentation so that (a) you will understand what the code does, (b) everyone else following you will understand what the code does, and (c) the developers themselves will understand what the code does, because sometimes you find bugs just by trying to figure out what the code does.
That being said, (warning: personal opinion ahead) I second the opinion that you should stay away from template classes. There are plenty of worthy numerical projects out there that do not rely on a maze of twisty little code fragments, all alike.