...although I'm not sure about all the free e-mail web based facilities such as Yahoo, AOL, MSN... but some, like Yahoo will not let you read you mails by using the POP/IMAP protocols (unless you have a paid yahoo-mail account).
But for all of you who have personal e-mail accounts (rather then web based e-mails service providers) and G-mail (they support it, I know because I've done it :) ), etc Popper will come in handy.
Remember, I haven't used it so cannot go into all the details... but if you want to give it a try then you can install Popper in Ubuntu 11.04 by using its PPA channel. Open your Terminal and enter the below command.
sudo add-apt-repository ppa:ralf.hersel/rhersel-ppa
sudo apt-get update
sudo apt-get install popper
By default it'll start itself automatically while your desktop loads (it should!) and for some reason if it does not, then you can manually add it to your startup apps with ease. But before using, you must make sure to set-up your POP/IMAP protocols. I also humbly advice you to read the developers page before start using it (if you're new to POP and IMAP that is).
For G-Mail users, POP protocol related information can be found in here and IMAP in here.