It's a little outdated now, but when I first was looking for the 770, I came across this.
Archos PMA

Sounds interesting. Same specs as the 770, except the OS isn't open source, although it is linux. It also has a 30gb hard drive, 8 hrs of battery life, and it can record live TV. It's $150 more than the 770, but that's not bad for 5 more hours of battery life, and a 30gb hard drive.
The main reason I didn't end up getting it was the lack of support from the open source community. There aren't any great sites like itt, maemo, or nokia770 for the archos.
Still an interesting competitor.