http://viewerdirectory.secondlife.com/ More information about Second Life and third party viewers and the third party viewer directory.
Policy on third pary viewers: http://secondlife.com/corporate/tpv.php
Emerald viewer is great for explorers, but I have heard many rumors & bits of information about it that if factual are very concerning. I realize I need more information. It would probably be safer for me to just stick with viewer 2, but I have been struggling to conform and learn enough to be completely comfortable with this very different viewer as yet. It's my understanding LL has made a list of sanctioned viewers and any viewer not approved or on the list by April 30 will be banned. This is in part to prevent content theft I'm told. I need some simple explanation of all this really. In the mean time I think my computer will continue to hold several viewers. I want to do the right thing, so if someone has researched this and can explain in simple terms, please let me know.
Addendum: Now I hear Emerald is working on a rebase Emerald that will satisfy the Lindens, then a new Emerald based on Snowglobe 1.3 with backport features of SL2.0 without the interface. More rumor? IDK