Here are some links to all kinds of sites - viola stuff, string quartets, places to order music and CDs. They'll all open in new windows. Have fun!

The Viola Web Site
A compendium of all things viola!
American Viola Society
They have student memberships as well - take a look!
Viola Jokes
Though not as popular as they were in the '90s, viola jokes still proliferate in musical gatherings, so if you want to know the punchlines, better check these out.
The International Music Score Library Project is a humongous time-suck, but a fantastic archive of music in the public domain (i.e. no longer under copyright), all available free as PDF downloads.
Colorado Quartet
An award-winning all-female quartet that happens to be some of the most fabulous people I know. They run Soundfest, a summer string quartet festival on Cape Cod that I highly encourage all my qualified students to attend!
Miró Quartet
They've got awards coming out their ears, they're amazing players, and they're a ton of fun. What's not to like?
Ying Quartet
Those of you with string-playing siblings, check these guys out - an entire quartet of them(or it used to be - they've acquired a new first violin, it seems)! Also my favourite coaches in college.
Orion Quartet
Those of you who continually wear mismatched socks (you know who you are...) should get a kick out of these guys... the rest of you will enjoy it too.
J. W. Pepper
A good source for sheet music.
Shar Music
If you can't find it here, it probably doesn't exist.
Johnson Strings
Our excellent local string shop, where you can get everything from an emergency string replacement to whichever etude books I just told you you need. My highly-recommended rental shop.
Strings Magazine
A subscription to this will keep you up-to-date on things in the string world; an excellent recent article on shopping for your first good instrument is here.
The Strad
The British string-player's magazine, widely available in the US as well.
Viola on Wikipedia
When you need a quick factoid or a bunch of random links, this is the place to go.

There are tons more, so go off and explore. A good place to start a hunt is Google.