Check out "myAppFree" for Windows Phone http://www.windowsphone.com/s?appid=286ba4a5-33c3-489a-a5a1-8675503aadca. It basically gives you a new app everyday that is normally priced, but is free for a limited time. I saved more than $10 with it. Keep in mind you might not always get a useful app.
Most useful(my opinion):
1.Nextgen reader
2. Pocket file manager
3. Opera mini
4.5translate
5.Accuweather
6.Tapatalk
7.Stop n go
8.Sofascore live score
9.Calculator?
10.Uc browser