Here are a few of my new found favorites:
www.ancestry.com - While this is a site you have to pay for, I often use it first because searching is so quick and easy. It all started last year looking for birth parents and has led me to research my family tree along with at least 4 other families.
www.familysearch.org - An amazing, FREE site! I have found birth records of German ancestors, death certificates which gives me names of parents and countless more information.
http://virtuallibrary.cincinnatilibrary.org/VirtualLibrary/Default.aspx - WOW! I am speechless about the amount of information on this site. The availability of searching Cincinnati City Directories has opened up a whole new world of researching old and/or historic buildings in this fair city. I just wasted away about 30 minutes of my afternoon just "paging" through the "Boyd's Handbook of Cincinnati", printed in 1869, found under the "Old and Rare Books". BEWARE - this site is dangerous to one's free time!
These are my current "go-to" sites, but then there is also just a good ol' fashioned Google search that can be amazing to what you can find.