Edexcel and OCR also offer the EPQ.
I have supervised students using both of these specifications. I loved working on EPQs - students developed fantastic skills that were of genuine use to them.
EPQs can be academic-style dissertations where you learn to analyse sources, do academic referencing, write abstracts and so on; or they can be the creation of an artefact, performance or event.
Whichever route you choose, as the post above points out, you'll have to show skills of project management, independent working, initiative and the analysis of a range of sources of information. Every student needs to keep a log or diary to track progress and provide evidence of their skills. At the end of the project everyone gives a presentation about their work.
In terms of marking, there is more importance attached to the process - the way you worked - than the product itself. So lots of stuff can go wrong and you can still score a high mark.
Basically you get to study something you're really interested in and develop lots of useful skills. It's a great qualification imo.