Sprint offers mifi, but not a pay-as-you-go plan (as far as I can tell).
I'm going to be driving cross-country and moving later this summer. Virgin's $60 for 5gb (expires after 30 days) would be perfect for me. Although it's not the most cost-effective option, I really don't need a two year contract since I'll have ready access to wifi at home and on campus nearly all of the time. I just want 3G for when I'm traveling--which isn't very often.