It's not free but ancestry.com is awesome. Familysearch.org is a free site run by the LDS church.

I use Ancestry most of the time. They have a free site -- Rootsweb but be careful if you use it. A lot of the info there hasn't been documented. I prefer to use the US GenWeb. Here's a link: http://usgenweb.org/states/index.shtml You can click on your state and from there check on individual counties.

Our local library has a subscription to "Heritage Quest". It can be accessed (free) from outside the library through the library website once you sign in with your library card number. It has a good collection of original image census data. You might check with libraries in your area too see what they have available.

I spend a lot of time on Ancestry.com and find my subscription there to be well worth the money. Of course you have to be careful with the user-submitted data (just like the problem Mady pointed out about Rootsweb - it hasn't all been documented) but there is a lot of original source material available. I subscribed to Footnote (www.footnote.com) for their Revolutionary War Pension images & Civil War Pension index. I don't find myself using it enough to be worth renewing my subscription, though.