I second "Find My Music Too." Someone here mentioned it and its great! The best player I've found.
"Reminders" by Michael Lg is also an app I can't do without. It doesn't rely on location so if you turn off Wi-Fi, location, etc, you still get the timed reminders. I use this app everyday. It's also nice to use as an alarm clock/timer to remind you to leave so that way you don't have to worry about an alarm going off the next day if you forget to turn it off (I really wish the alarms would silence when you silence your phone).
http://www.windowsphone.com/s?appid=bd3986b9-09d8-4181-8e4a-428cf65c6aa3
I really like the Evernote app, if you use that. It's pretty nice and you can pin notes and notebooks (which for some reason OneNote won't let you do).
I also downloaded Cal, but really just for the tile. I use the default calendar app to add things, but I like being able to see multiple future events on the live tile.