Linden Labs Viewer Policy and New Viewers
Linden Labs has created a new policy about third party viewers. Basically, viewers that does not respect their Term Of Service policy or does export items without respecting the permission system will no longer be able to login to Linden Labs's server. This policy is enforced starting on 2010-05-01. For detail, see: secondlife.com Policy on Third-Party Viewers.
They also have a page listing approved viewers at http://viewerdirectory.secondlife.com/. Currently there are 7 third-party ones. Some of these viewers have interesting features. For example, a text based viewer, viewer for cellphone (iPod, iPhone, Android), viewer for film makers and photographers (basically high-end rendering).
Note that Emerald viewer is compliant. I highly recommend the Emerald Viewer for power players.