One of the sites I frequent when doing flight prep is AirNav.com. Though you're probably already familiar with it, AirNav provides free detailed aeronautical information on airports and other information to assist pilots in gathering information for flight planning.

The site is also useful for some hangar flying on those days when the weather or the checkbook keep you on the ground. Airport details include airport location, runway information, radio navigation aids and communication frequencies, FBO information, fuel prices, and a wealth of other information for pilots.

It's an indispensable site for my use, particularly airport and FBO information. I highly recommend it.