You don't have to change the panel on a dedicated server (or VPS) just to add a webmail client. You can install whatever software you like.
But the software you install outside of cPanel usually won't automatically update with the stuff cPanel installs by default, so you'll have to manually check for updates (or set up a cron job to fetch the updates).
There are tons of open-source webmail apps. You can find a bunch of them at
http://sourceforge.net/search/?type_...ords=web mail
There are also commercial ones, which may (or may not) be better supported.
Of course, whenever installing software, make sure to consider stability, resource use, security, and so forth before making your decision.
Frankly, I don't think there's much difference between webmail clients, and so few of my clients use it that I don't bother installing anything special unless they ask (and pay) for it. It's also possible to customize either Horde or Squirrel Mail to some extent if you want to spend the time. But if a pretty or more full-functioned webmail client is important to you, there are other options available.
Best,
Richard