Thunderbird is very powerful, and allows you to create folders locally and remotely. It supports extensions (including the up-and-coming calendar application Sunbird). It also allows you to read newsgroups and RSS feeds. The newsgroup support is good, but the RSS feeds could be better.
Thunderbird supports IMAP and POP3, as well as SSL and GPG (via extension). Bayesian filtering of email is also built-in. I've used it extensively and rely on it exclusively for my electronic mail both under MacOS X and Windows. It's multi-platform support is very nice, and allows you to be in familiar surroundings no matter which OS you are using at the moment.