I don’t know of a site that catalogs them directly, but one strategy that I’ve used on occasion is this:
- Find an old, small, unmaintained open source project on sourceforge
- Download it, get it to compile/build/run
- Read the documentation, get a feel for the code
- Use the techniques in Working Effectively with Legacy Code to get a piece of it under test
- Refactor that piece, perhaps fixing bugs and adding features along the way
- Repeat steps 4 through 6
When you find a part that was especially challenging, throw away your work and repeat it a couple times to reinforce your skills.
This doesn’t just practice refactoring, but other skills like code reading, testing, and dealing with build processes.
The hardest problem is finding a project that you’re interested enough in to keep working in. The last one I worked on was a python library for genetic programming, and the current one I’m working on is a IRC library for Java.