Tiff gave me a mango drink with caffeine in it, and therefore...
I couldn't sleep
...and wound up getting up at 2am (went to bed at about 1 or so), then going back to bed at about 3 or so, then getting up again after an hour or two. I was pottering about on the memorial island (where I ran into a great area protected by banlines, I hope mistakenly), and when I left there and checked the forum I had a pm from Boy asking me to assess the two newest shadow viewers, so I did, with results that might be useful for comparisons, and if not - what would anyone expect since I haven't had any sleep.

Viewers to be tested against one another:

In this corner we have the brand-spanking new SD17, which incorporates the viewer changes of the S16 along with shadows.
Pros: I'm grateful KirstenLee compiled the first viewer with shadows that I ever used.
Cons: Non-compliance with the dictates of open sourcery (and we all know why).

And in the other corner we have the still sparkly-fresh Shadow Viewer by Boy Lane.
Pros: Boy complies happily with open source dictates. She creates viewers (notably the CoolViewer) just for SL, and is enthusiastic and open in her contributions, as well as a constant presence on the forum asking for feedback.
Cons: None I know of.

Puter (was big when I had it built two years ago but is medium spec now)
Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 3
Processor: Intel(R) Core(TM)2 CPU 6700 @ 2.66GHz (2 CPUs)
Memory: 3326MB RAM
Card name: NVIDIA GeForce 8800 GTX
Chip type: GeForce 8800 GTX
Display Memory: 768.0 MB
Current Mode: 1680 x 1050
Driver Version: 6.14.0011.8250

Environment
256 draw
Midday
Standing
Ultra settings (everything maxed)

Note: FPS waddles around and I settled on the whole number I saw most consistently.

Far Shot

SD17 ^^Shadow Viewer ^^
--------------------------

Close-Up
SD17 ^^Shadow Viewer ^^
--------------------------

SnapshotSD17 ^^Shadow Viewer ^^

My Conclusions Based Upon Results Which Might Well Be Very Specific to My Hardware/Location/Choice of Dress

SD17 = ~17fps
SV = ~20fps <---Winner

SD17 = Borked Snapshot
SV = Snapshots <---Winner

SD17 = alpha glitches (inconclusive; to be expected)
SV = alpha glitches (inconclusive; to be expected)

Fair Play Award = +1 point to the Shadow Viewer for Open Source compliance.

Based on this entirely unscientific comparison fueled entirely by a sleepless night, I recommend the Shadow Viewer by Boy Lane, to be obtained by clicking this link HERE.