I recently found a Four Thirds Leica 25mm f1.4 + M4/3 adapter on eBay for $600. This is the only auto-focus prime available for the GH1 or GH2 that has a manual aperture ring. It's sharp at f1.4 and auto-focuses in low light nearly as fast as a native M4/3 lens.
I also have a Canon FD 28mm f2 that I picked up for around $200. While it's not as sharp as the Leica and I wouldn't use it for stills, it has that organic FD look that works so well for GH1 and GH2 video. There's also an FD 24mm f2, but it's usually priced higher than the 28mm f2. The affordability of the vintage FD prime series was what originally sold me on the GH1.