I use many web mail clients, most of which allow retrieving from other accounts using POP3: Yahoo, Gmail (mentioned above.) The new kid on the block is GMX (www.gmx.com), which is pretty slick - very nice and responsive interface, built-in support for grabbing e-mail from other webmail services (I use it to grab my Hotmail e-mails), a 1Gb file storage area and a few other things. Mail.com's web interface isn't great, though they're slowly implementing changes lately and it's getting much better.
I use Roundcube for checking my domain's mail account remotely, but find it horrible. Very often, this inbox will display "ghost" e-mails - i.e. e-mails that I retrieved the day before using my standalone e-mail program. Click on the e-mail and it's not there. I think CPanel offers SquirrelMail (mentioned above), I should try that if only to retain some sanity. :)