This is a great program for WP development. I mostly use it to browse Isolated Storage. Windows Phone Power Tools - Home To be able to use it in Windows 8.1 it is a thread with instructions in the discussion on the page.
There's also Metro Studio, which is another great database of metro icons, and you can create your own! Lots of flexibility within the program, and it looks nice too. Also, all of the icons are free, and you can do whatever you want with them!
Personally, a decent photo manipulation program (GIMP, Paint.net, etc.) and Metro Studio are good enough tools alone to create icons of almost any icon required...
And if you want to beta test your app, and I'm sure you do, you can use this service to find new beta testers as well as easily manage your beta testing process.
Well, if your planning on creating Web API based applications, the website ProgrammableWeb provides an entire database of all service web/mobile API's. I also use Stack Overflow which is a great forums website.