1) Windows Live Search

Windows Live Search on a mobile  is better than Google search on a PC. You can browse by Category based on your location and follow that up with map based directions and route planners.  Very impressive.

Download on your phone via: www.wls.live.com

2) Google Maps

Google maps is another great free map application It has many options like looking up your contacts, getting directions etc. Although some of what is does can be done by Windows Live Search you may as well have both applications installed, neither take up that much room.

Download: http://www.google.com/gmm/index.html

3) TCPMP The Core Pocket Media Player

This can be used to play back multiple audio/video formats like avi/xvid for example.

Windows Mobile devices can make a great portable media players, particularly the ones with the larger screens.

Download from: http://www.pqdvd.com/download/tcpmp.setup.0.66.exe

4) Google’s java based Gmail client

If you have a Google email account this Java application supplied by Google is the best way to access your Gmail account on a Windows mobile, the user experience has been designed from the ground up to work within the constraints of a small screen.

Download: http://www.google.com/mobile/mail/index.html

I will update this post if I find any other, if you want to share some your suggestions please use the comments section.