What Rob said, plus...
What does ForeFlight cost and what does it give you?
Oh, approximately everything.
Live (weather) data: Metars, TAFs, winds aloft, radar, satellite, NOTAMs (organized by issue date, so you don't have to read any of the ones you already know about), DUATS briefings, all of the NOAA graphics (the stuff you'd find on aviationweather.gov), TFR maps, and a few pieces of data from Weather Underground. (That's for the USA, they also have international data.)
Downloadable data (28-day cycle): All A/FD data (Both US and international!), AOPA airport directory data plus some additional supplemental data from other companies, Sectionals, Low and High enroutes, approach plates, airport diagrams. The A/FD and AOPA data is always downloaded, while all of the rest of the data is selectable by state for automatic download, and is otherwise available for on-demand download. For example, I have it set to download all of the approach plates for Wisconsin on every new cycle, but when I bring up an airport outside Wisconsin, I can either individually select plates to download (as I view them, they're cached until the next cycle), or I can hit one button to download all of the plates for that airport. It's very well-done.
Finally - Additional functionality:
* Flight plan filing - Including alerting you when the plan is actually successfully filed with the FAA
* Flight planning - Just punch in "KFCM 3M0 180kts 26gph 8000ft" and it'll pop up a nav log. (You can put waypoints in between as well.) It'll also show your route on the charts, or even the satellite/radar maps. Routes can be stored for future use too, and you can set a default airspeed and fuel burn so that you don't have to type them in each time.
It's one heckuvan app - Very well-designed, tons of features, easy to use. Well worth the price. Download it and try it out!
OBTW - We talked to Tyson Weihs and Jason Miller of ForeFlight on Pilotcast 72 which is now live - Give it a listen!