Firefox

Looks like the popularity of firefox is really taking off.
Get Firefox There are some fantastic extensions such as the Web Developers by Chris Pederick which is a must have for anyone interested in web development. Others that I have started using include the "Mozilla Calendar" and Sage a lightweight RSS and ATOM feed aggregator.

If you are still using IE and Outlook then it is really time to do something about it, there have been plenty of articles recently pointing out the security faults of IE.

Get Thunderbird is an excellent mail client without the security problems of Outlook.