I use Thunderbird. I like it because it supports multiple e-mail address, has import and export of your address book in various formats, offers a version that runs completely from a flash drive. The later is especially useful if you travel without your laptop but have access to an internet connected computer with a USB port. All you need is your Thunderbird configured flashdrive. Allows you to read your e-mail using the same program you are already familiar with. And by configuring your flash drive version of Thunderbird to leave your e-mail messages on the server, when you get home you can download them a second time, archiving what messages you want to permanently save. Thunderbird also integrates well with free PGP based encryption. This allows sending and receiving e-mails that can only be read by intended recipients. It's text editor allow for easy switching between plain text and HTML message composition. It also has a easy automatic upgrade feature. Very useful for closing the security holes when they are discovered. The only thing I don't like about Thunderbird is the so so quality of the integrated spell checker. Overall it works well for me, and it's free.
One of my free e-mail accounts is gmail. It allows POP server clients. Many of the other free e-mail accounts are only Web based. The POP server configuration downloads all your new messages a once. You don't need to stay connected to the internet to read and compose messages. Very useful when you have limited connection time available, but still need to get all your e-mails.