Congressional staffers maybe? I've read low level staffers make about $30 or $35K (in the DC Metro area to boot), working awful hours for generally terrible people (my guess is only those with monstrous egos become congresspeople). I imagine many are sharp, hard-working, ambitious people. I guess they do it to be "near the action" or something.
Any job with lots of turnover would meet most definitions of underpaid (provided that they're leaving to make more money, not because they've been fired). From google:
Nurses
Child-care workers
Accounting
Telemarketing
Movie Theater employees (doesn't sound like a bad first-time job, who knew?)
Hotel/Restaurant
Sales.