1. Gmail Java app for mobile phones

http://www.google.com/mobile/mail/index.html
Very useful for your everyday needs, calendar, email, contacts and more.

2. Google Maps for Mobile

http://www.google.com/gmm/index.html
Very easy to use and works well with your GPS, maps are clear and there is constant data updates. A real competitor to Nokia's attempt.

3. Opera Mini

http://www.operamini.com/
A lot faster then Internet Explorer Mobile and a lot easier to use, they have really concentrated on every detail to make the software really easy to use.

4. Fring

http://www.fring.com/
This handy little lightweight application is the best I've found for IM usage. The speed of the application is instant and it supports Skype (chat and calls) and kicks ass over the Skype Mobile application. The application supports the following services:-
1. Fring
2. Skype
3. MSN
4. Twitter
5. and more

5. Shozu

http://www.shozu.com/
Nothing worse then having to copy your files to your notebook/computer after you've spent the day taking photos on your mobile. Shozu allows you to simply browse your photos/videos and select which files you want to upload to your account (Flickr, YouTube etc).

For more details, check out Read/Write Web