This one is not in Interlaken, but close. It's called Mountain Hostel. very friendly staff, cheap laundry, free internet. Great views of the Jungfrau, heaps of day hikes, bon fires, hot tub. The other places to stay around this village seemed nice as well, perhaps a bit more expensive (MH is 20 franks a night!) easily the coolest hostel I've been to. You can't go all the way to interlaken and not spend at least 3 days in Gimmelwald (not the touristy town of Gimmelwalt).
I would have stayed longer if I didn't have flight reservations. I'll post pics tonight here if I'm not dead on my ass from work and school.