I have devoted much time to ranking the apps properly, but I use and like these ones:

1. FT Web App - free
2. Guardian - the app is free, but it's approx £4 a year to access content
3. WeatherPro - not free, can't remember the cost
4. Twitter - free
5. C4 News - free
6. BBC News - free
7. BBC iPlayer - free
8. 4oD Catch Up - free
9. Amazon - free
10. Remote - free (for use with Apple tv: changes my ipod touch to a remote control)
11. Link to the TMF boards :-)

I'll mention that I have the facebook app, and that I use it, but it is rubbish, so while it would make it to the top ten in usage, it wouldn't in likeability.