Lets get this out of the road right up front...I'm a tight ass when it comes to spending money on development resources. I'm still dipping my toe in the water to get a feel for the WP7 platform and trying to gauge the effort / reward ratios and as such don't really want to drop a heap of money to produce something. So below is a list of resources I've used or intend to at some stage for my game Apple Bin. Hopefully this will be useful to someone, if you've got anymore feel free to list them in the comments.
Having a custom font for your app really gives it a unique style. Font Squirrel has some top quality fonts, all you need to do is find the one for you!
There are a fair few free graphics sites out and about. I've found this one to be good as a starting point for a lot of vector graphics. I think I've got about 4-5 elements from here but have altered them before putting them in Apple Bin
Kevin McLeod has some amazing tracks suited to games of all shapes and sizes. The interface is a bit clunky but stick with it and listen to the tracks on there and i'm sure you'll find something to suit your game or app.
Lots of great sound effects in here. Grab Audacity to strip down the audio and customise it.
I've done a little tutorial on my blog on this tool for getting a video onto YouTube. I think it's an excellent tool for getting something together quickly and easily.
Game High Scores
This is a free service, I've always intended to incorporate this but have never got around to it. From all reports, easy to use and did i mention it was free? :)
Based on the box2D engine, really active community and some really talented contributors.
Allows you to use the Farseer engine and setup everything in blend. This is what I've been using for Apple Bin and have found it great.
*Not free but awesome
Check out the artist Soundroll in here. Tracks are about $10-15 but they all top quality.