Lists
There are all kinds of viewers now, and certain innovations are just on one or the other: no viewer at the moment has all of them. I'm trying to think of the ones meaningful to me:
V2-based viewers:

• Katharine Berry's shareable WindLight assets (in Kitty and K-Lee)
• Depth of Field (newest Mesh project viewer)
• Mesh (Mesh project viewer)
  
• Qarl's Align tool (Ascent, Imprudence Experimental and maybe other TPV by now)
• Export to xml (Imprudence, and other TPV)
• Grid Manager (all TPVs I believe)
  

What else? There are more, I think, but that's all I can think of at the moment.

There are more viewers now than I even know, I think, but not all are authorised to connect to SL (and so are used for OpenSim grids). Viewers include:

Official: V2, V2 Beta, V2 Mesh, Snowstorm, v1.23
TPV: Imprudence, Ascent, K-Lee, Phoenix, Emergence, Kitty, Hippo, Dolphin, Restrained Love, Inworldz, and others
I fear Meerkat is defunct.
Browser-based viewers are beginning to show up, including Spoon's cloud runner, the fruits of the Skylight project, and IBM's Canvas for low-spec biz/edu-puters.
Pocket Metaverse runs SL (purportedly) on iPads/iPhone.
There are light viewers like texty Sparkle and Metabolt but I've never used them.