I'm a fan of Radegast, a lightweight SL viewer that pares down graphics and other functions. I use it when I just want to do something simple like clear messages or chat in IM (especially if I'm on my backup computer), but some artists (like Jo Ellsmere and Oberon Onmura) use it to run bots that are part of their work. Radegast was orphaned when its developer passed away, but fortunately someone volunteered to take it over and it's alive again. (Thank you, Cinder Roxley!)
A few ago, Norton Security stopped me from using it by cutting off its access to my internet, thinking it's a malicious program. After a few days of this, I was pretty annoyed. So I made a "false positive" report to Norton and -- hurrah! -- they just told me they'll take it off their blacklist. Might take a day or two for that to appear in their updates. Lesson: Complain, because sometimes it works.