New databases are added each day and can be accessed for free for the first 10 days they appear. Genealogy Library - also one of the leading sites for source information. Plus they have actual images of census records which can be viewed for a fee. LDS Family Search - can't forget probably the most popular site. The Mormon Church has created this site from their ever popular CDs.
Access is free.
For a list of the programs available on the market, and links to them, check out the Software section on Cyndi's List . Some of these programs are shareware, so you can try them out then buy them. Some have limited demos. And there are a few free programs available as well. Family Tree Maker is used to maintain the database in this site.