There isn't. An Amiga wouldn't have the horsepower to run it.
Well, an NG-Amiga might. An X1000 or an olde Mac running MorphOS would be powerful enough for a decent SL-experience.
Theoretically you can create a SL-Client that doesn't show much of the GFX. You might even create one that runs stable and usable on a real Amiga, but I guess you wouldn't see much more than green dots on a map.
But it isn't worth the trouble. Practically noone is in SL anymore. I checked today - the places are all deserted.
You need some virtual company on an Amiga? Try IRC (e.g.
http://wookiechat.amigarevolution.com/wookiechat/) instead!
