I’m not a tools guys… really I really only program to be able to design so tools and I don’t mix but unfortunately the devils must… XNA has removed a large portion of the hell of writing DCC importers which is nice but still misses out the other vital tool IMHO.
A level editor
The last time I attempted one was about 10 years ago in MFC and DirectX5 IIRC I’ve worked on bits of in house editors (Sudeki Ed, MrEd (HS’s), UnrealEd etc.) but haven’t started from scratch but really felt I needed one so figured time to knock one up. I thought this was gonna be a huge soul sucking experiance but I guess that shows how little I’ve programmed with modern .NET gui.
.Net & Forms is good :D in about a weekend, I’ve got an editor that reflects in the game and using a few interfaces allows you to add enties and alter any properties of your entities in a fairly swish UI. Using my XNA renderer into a panel I can display and move things.
I’m amazed that in about 2 days worth, I’m about half way to hitting my basic editor I wanted (just hit testing, a few manipulators and serialization til its rudimentary useful). Don’t get me wrong thats purely the most basic rubbish editor you can imagine but still for the amount of work thats amazing.
 Forgot to mention, Ziggyware article has the details on was what i based my form XNA intergration. Here
So heres a piccy and a link to it fullsize