Without a doubt, the paid online genealogy tool I use most frequently is Ancestry.com. But who hasn't already extolled the virtues of that website?
So I'll list some of the other paid sites that I use regularly:
GenealogyBank - Historical books, newspapers, documents dating back to 1690. You can search by name or by topic (e.g., "147th Infantry"). Careful ... it's really easy to get sidetracked here.
Fold3 (formerly Footnote.com) - mainly used for military records now, but it still houses all the records it had before. You just have to go looking for them. One feature I like about this site is that you can create pages for people, events, groups, etc. and link documents you find to those pages. You can also download the documents to your computer.
WorldVitalRecords - from census records to land records to newspaper archives to yearbooks and maps ... you just never know what you might find here. Start with a broad search, then narrow as you go. Caution: also easy to get sidetracked on this site too!
I just hope there is an upcoming topic for free sites. I use WAY more of those (and sometimes with more success than the paid sites).
Disclaimer: I haven't been paid to name any of these sites, particularly Ancestry, which is why I didn't want to put a commercial for them on my blog.