Nokia has an app called App Highlights, its in the Nokia collection section of the market and its a good place to start.
Yup, it's actually my fav. You can filter by rating, price, and category.
My fav apps:
Amazing Weather HD: excellent weather app, live tile and lock screen support
App Highlights: helps app discovery and easy to find/filter/search apps
Battery Status: battery app
Connectivity Tiles: shortcuts to wifi, airplane mode, etc.
Facebook
FFFFound: live tile and lock screen support, finds random art from the FFFFound site
Flashlight XT: no frills flashlight app
Nokia's HERE apps: Drive is most important here for me, great navigation, offline maps
IMDb
LastPass
Mango Transit: HERE Transit doesn't support DC
So I use this
Metrotube: Best Youtube app
Newser: Great news app, live tile and breaking news alerts. I choose this over something like Weave because it shows the headline on the live tile, not just the image
Nextgen Reader: Best Google Reader app
Office
OneNote
Pageonce
Photosynth: excellent 3D panorama app
Podcast Lounge: best podcast app, IMO--WP's podcast support is bad
Rowi: good twitter app
SkyDrive
Skype
Speedtest.net
Talking Alarm Clock: I use this to turn my phone into a clock/alarm clock at night
TVShow
Vimeo
Wikipedia: GREAT 3rd party wikipedia app (yellow icon, if you have trouble finding it)
WPCentral: WP news source
YouTube