Native Second Life on both 32bit and 64bit Linux
Years after Second Life hit the scene there is still no good stable Second Life client for GNU/Linux which is a damn shame, especially given that Second Life takes great advantage of Linux by running it's server farms on Debian. Be that as it may, I just found out about a project which may help rectify the situation sooner (or rather, finally): Open Metaverse Viewer. It's purpose is to "package the Second Life(tm) viewer for Linux distributions" and it already provides repositories for Ubuntu, Debian, Arch and Gentoo.
There's also an option to build from source.
Given that this viewer is based on Second Life client, which is still in beta, great stability probably can't be expected, though I'd venture to guess that if you've got a good working 3D driver for your video card, it will probably work well. I tried it on mine, which isn't very good (free Xorg driver) and it works, but tends to crash and freeze the whole system almost randomly. If you have an older ATI card and are stuck with free drivers (since AMD just dropped support and left jaunty users in cold) safest bet is to run at lowest settings. Good luck.