Black Dragon, the third party Second Life viewer with a cult following due to its amazing graphics and fast frame rate -- we're talking up to 100 FPS -- now has a 64 bit version that's reportedly even faster, creator NiranV Dean just announced:
Please note that this 64 bit version is absolutely the same as the 32bit version except that it is now obviously 64 bit. From my tests so far however I can tell you that it runs a lot faster (for me) than the 32 bit version ever did... make sure that texture loading is done before comparing your FPS, while textures where still downloading I got roughly the same low 10 fps but when they were done loading I went up to rock solid 25-30 fps. That's with deferred, shadows and SSAO on.
64 bit version available here, more info on NiranV's blog which (NSFW warning) is full of graphically nude furries (because of course it is), but you can also talk with NiranV on this Reddit thread where the nude furries are opt-in.