I didn't see one of these threads in the first three pages of this board so I thought I'd start one. Remember to link to the http://www.windowsphone.com/ entry and note whether it is available to only certain versions of Windows Phone.

The ones I like so far include:

Geekzone

- WP 7.5+
- Remember to set an API key (in your profile) so you can view the forums.
- Read only for now.

Subway

- WP 7.0+
- I love this app despite not regularly eating Subway.

My 2Degrees

- WP 7.5+
- This app works but isn't amazing. Definitely very useful though :)

AirNZ mPass

- WP 7.0+
- Excellent little application, has a few things that need to be fixed but overall very handy to replace your boarding pass.