I tend to use Mozilla programs for browsing and email, which I can highly recommend. Firefox is an excellent browser, especially for web development. Thunderbird is an email client with a very rich featureset. Both can support plugins or add-ons, which can change them from being “useful” to being “indespensible”.
I've only written one plugin for Thunderbird so far, but more might be on the way.
As of version 2, Thunderbird still forces you to go through an awkward process of opening a properties window if you want to be able to check a folder for new items. If you have a large number of folders, this quickly becomes very annoying. FolderCheck adds a new item to the context menu for folders that allows you to quickly and easily toggle the “check this folder for new messages” setting.
More information on this site or at Mozilla's add-on repository.