When working on our startup, we used to use Assembla solely because it was free. After we moved past the whiteboard phase, we switched to Unfuddle immediately. The productivity gain alone... If your site is awesome (like GitHub), peopel will pay for it, even students. The bigger problem (for us at least) was not lack of funding, but lack of a funding method. Source control should really be a business expense. Sure, everyone can kick me five bucks or something, but that tends to make accountants mad.