Obviously this is not an exhaustive list of online sweepstakes resources - but just a few to get you started.

The biggie of course is OLS, or online-sweepstakes.com, which has been discussed here before. While it does have some free listings, the good sweeps are accessed by subscription only: http://www.online-sweepstakes.com/

I don't know much about the following, so you're on your own:

CashNet Sweepstakes http://www.cashnetsweeps.com/

FatWallet http://www.fatwallet.com/

Sweepstakes Advantage http://sweeps.ws/