If your graphics card is up for it Second Life Shadows are pretty cool. But be forwarned...even my 1GB 8800 Nvidea card has to be tweaked a bit and performance is really degraded. Of course this computer "only" has 2GB of RAM.
What's cool is that the shadow to the right of me is cast by a prim that has a texture that has transparent regions and that is reasonably well echoed in the shadow! How's that done?