Since we moved on to opensim, but are still present in Second Life - the page is split.
For OpenSim
The most popular free grid http://osgrid.org .
The Server software http://opensimulator.org .
The most popular multi-grid viewer http://opensim-viewer.sourceforge.net/ .
For Second Life:
Where to get the software ?
http://secondlife.com/community/downloads.php
How is the grid status ?
What is going on, are my problems allready known ?
Where do i get into my account (money, land, membership...) ?
Where do i get help ?
http://secondlife.com/community/support.php
one of the best places for sure is the forum-system: http://forums.secondlife.com/
in-world: use help request, report bug or the neebie-places like where you´ve been transportet first