Thunderbird is a neat email client that has a small footprint and features some smart ways to stop junk mail.
You can delete junk mail straight away or place it in a folder for reviewing later. The filtering mechanism lets you mark junk that may have been missed, and it will use Bayesian filters to learn over time.
It is possible to customise the toolbars, removing and adding buttons at will, and change how the program looks with themes.
All the popular mail formats are supported and it comes with options for importing mail, address books and settings from Outlook, Outlook Express, Eudora and Communicator.