This is an awesome thread, I hope more people join in here. This is a great way to learn about what is out there and works. I can't believe some of the apps people come up with!
My Favs-
-Our Groceries (free)
Allows me and my wife to easily sync a grocery/shopping list on our phones.
-Netflix (free app with paid service)
No need for explination...
-Shazaam (free)
Still use this all the time to find out what that awesome song is on the radio when I don't have time to wait for the DJ to tell me.
-Minus (free)
Like dropbox but you get 10GB of free storage right off the bat.
-Root Explorer (paid)
The best file browser/manager I have used to date.
-Groupon and Living Social (both free)
Great deals and easy
-TuneIn Radio (free)
Streams all my favorite local radio stations over 3g or wifi no matter where I am.
-MyDays (free)
Keep track of my wife's ovulation, menstruation, and likely PMS days. A life saver!

-PureGrid Calendar Widget (free)
Very nice customize-able calendar widget that syncs with my google calendar.
-Barnacle Wireless Tether (free) and PDANet (free/paid)
Best tether apps out there.
-Documents to go (free)
For when I need to view pdfs, powerpoints, and word docs.
-Phandroid app! (free)
Gotta have my access to this forum easy and available at the push of a button at any time!
-XDA Premiu (free/paid)
Another GREAT forum!