Thunderbird is an incredibly light desktop email offering POP3 and IMAP accounts and a feature-rich interface. It's memory footprint is a fraction of that of Outlook and Outlook Express, and thanks to being a Mozilla open source project, is extremely customizable, with a vast library of plug-ins and tweaks. The only thing that kept many people using Outlook was the calendar function, which while a bit clumsy was definitely usable.
When the latest version launched without a calendar, many people were disappointed, but Mozilla quickly unveiled the reasons: Sunbird and Lightning. Sunbird is a cross-platform calendar and Lightning integrates that functionality with Thunderbird seamlessly. Just follow Mozilla's directions to install Lightning and you're ready to go.
But it gets better.