Aside from the standard integrated Google Calendar, the most useful app I use is COL Reminder
. If I have something that I need to be reminded of later (like "Pick up milk on the way home"), it's a very easy and lightweight way of doing it. I could technically use the calendar for that, but I don't need stuff like that clogging it up.
I also have the WAFF Storm Tracker
app, which pings me when there are severe weather alerts. Definitely needed in this part of the country.
The only app I've paid for is Sketchbook Mobile
, which is a very nice sketching/painting app!