After polishing and protecting my wood table at home and posting my tips for the job, Step-by-Step Basics: How To Polish Wood Furniture, I became a little more careful with my furniture. Here are a selection of coasters to protect your furniture in an attractive way.
1. Jet Set Coasters (48.00 from Jonathan Adler)
2. Stained Coasters ($19.95 from CB2)
3. Gem Coaster Set ($42.00 from Furbish)
4. Celestial Coaster Set ($32.00 from Anthropologie)
5. Spotted Coasters ($18.00 from Furbish)
6. History of Art Coasters ($20.00 from MoMA)
7. Toasted Coaster ($14.00 from Urban Outfitters)
8. Hand Crafted Reclaimed Aromatic Cedar Coasters (Price on Request from Brooklyn-based Tri-Lox)
(Images: As linked above)

Sheex Bedding
Toaster coasters - I love them!
We have these, which have been a lot of fun: http://www.suck.uk.com/products/bargamesbeermats/?category=gifts
I find that coasters with shiny/sealed surfaces don't work that well because they don't absorb liquids at all. Before you call me a messy drinker - I mostly have that problem with condensation on cold drinks on a warm day.
Wood and cork work best.
Price on request for a coaster. Pfffttt.
Those are super cute and all, but those prices are CRAZY!
The toast is adorable, but it looks like it doesn't have a backing, which means the liquid can go straight through it and still damage the table surface.
@HHRI, ditto. I feel like a coaster is there just as much to absorb the condensation as it is to protect the table. It's functional no-go if there's a shower of glass-sweat every time I pick up my beverage.
I resent the cleaning I have to do when I set my drink on the edge of a coaster and it yips over! It has put me completely off coasters, if there's a ring on the table I'll just wipe it off, no big deal.