At the "halfway through SICP" level, it's probably a bit early to start projects that are interesting. Do you have any pointers for finding open source Lisp projects that could use beginners?

Actually, halfway through SICP sounds like a great place to start projects that are interesting. For example, my first real exposure to Lisp was through Arc. The tutorial was enough for me to write a basic ray tracer (I submitted it to the unofficial arc repository a few months back).

I've never looked for open source lisp projects myself, but I'd give a site like github a try (just search for Lisp).