Family Tree Maker...and My Brother's Keeper is a good one too.
I sub to Ancestry.com, and have several trees there, I also find lots of good info on Footnote.com. FamilySearch.org is the LDS site, and while you can find tons of trees there, don't trust them, verify everything you find on any of the websites. If it's proven through documents, then it "ain't necessarily so".
Don't forget our old friend Google! I've gotten lots of information just by googling an ancestor's name.