Email Clients
I've used most of the email clients in my years on the web. Some are good, some are okay.
Probably the best all around free email client is Microsoft's Outlook Express 5.0. This client comes free with Internet Explorer 5.0 and it lives up to the usual quality of Microsoft (I know many people like to slam Microsoft, but I've found their products of late to be of high quality). The program is simple to understand and easy to use.
Personally, I use Outlook 2000. In my opinion there is no better email client on the market. Outlook 2000 does everything. My favorite feature is the rules (which allow you to specify automatic actions to perform on messages as they are sent or received), which I use to automatically file several hundred different email newsletters into separate folders. This saves me lots of time.
My recommendation would be to use Outlook 2000 only if you've purchased the Office 2000 suite. Otherwise, stick with Output Express.
Other email clients include the one that comes with Netscape - I found it to be pretty lame in comparison to the Microsoft offerings. Eudora is pretty popular and has lots of features. The free version is fine but if you want more features you can purchase a professional edition.