There are a lot of "afternoon tea" websites out there,
so I'm not even going to try to give you a comprehensive list.
Here are a few of the sites I especially like:

Visit Joy of Baking's English Tea Party page for a mouthwatering afternoon tea menu, some great tea recipes, and a brief history of tea time.

Check out The Tea Caddy.  This "purveyor of fine teas, teapots, and tea accoutrements" sells some absolutely glorious teapots.  (They have tea cozies, too.)  It's a very pretty site, and they have a listing of tearooms by state.

Crabtree & Evelyn, my favorite store for soaps, candles, and "bath goodies."  This isn't exactly a "tea" website, but C&E does sell loose teas and jams and cookies.  Their lemon cookies and strawberry preserves have often graced my tea table, and I use their blue-and-white "Mason's Ironstone" china every morning.  Bless 'em!

Visit Tea Time Worldwide for teas imported from England.  Don't miss the comprehensive "Tea History," the tips on choosing a teapot, and other interesting articles.  They also have a nice listing of tearooms all over the world.

Tea Time Traditions is a nice section of a larger site.  Good tips about afternoon tea service and etiquette.  (But please--ignore the part where the author gives you permission to use paper napkins at teatime!)

Take a look at Tea Spirit, which has a good listing of "afternoon tea" books and cookbooks.

Be sure to visit the Hosting an Afternoon Tea Party page at About.com.  You'll find a brief but very helpful discussion on everything from tablesettings to menu suggestions.

Teas2Dine4 is a pretty website with tea party tips, recipes, links, and lots of good content.

Here's a very well-done Scottish website filled with information and tips for planning a perfect afternoon tea:  All About Tea.

Want to plan a tea party for the ladies at your church?  Head to this helpful page: Creative Ladies Ministry.