This can be a bit tough to call, since some hardware reivisons are advertised, and some are not.

For example, while the Sega Genesis has had multiple obvious "models", there also tends to be variations in the specifics of each model that might affect things such as audio/video quality or game compatibility.

Anyway, for me this will probably be easiest to list the hardware revisions I know of for each system;

1) NES/Toploader NES/Sharp NES-TV Combo, Famicom/Sharp Twin Famicom/Sharp Famicom Titler/Sharp Famicom-TV combo
2) SNES/SNES Jr./Super Famicom/SFC Jr./Sharp SFC-TV Combo
3) Game Boy/Game Boy Pocket/Game Boy Light (and if you count these, Color, Advance and Micro lines, and Super Game Boy?)
4) Genesis Model 1/Model 2/Model 3/future variants based off model 3, X'eye, CD-X, way too many others to count (Genesis probably has the most hardware variants I know of)
5) Game Gear... uh, GG Kids? I dunno.
6) Turbografx-16. Most of the revisions were on the JP PC-Engine "Core Grafx" lines and whatsuch, I don't know much on them.
7) PC-FX has only one known model I'm aware of.
8) Sega Saturn has a few subtle model changes. I'm only well aware of the Model 1/2 ones, but I think there were others as well.
9) Playstation/Subtle model changes before PSOne/PSOne
10) N64... uh, more or less didn't change besides special casings to my knowledge.
11) Dreamcasts with Mil-CD/Dreamcasts without Mil-CD.
12) PS2/PS2 Slim/PS-X
13) Xboxes with sublte changes, mostly to try to break homebrew mods.
14) Gamecube/Panasonic Q
15) 360 Pro/Premium/Elite/Arcade/etc.
16) PS3 20/40/60/80gb models, upcoming PS3 Slim
17) PSP, PSP Slim, PSP Go
18) DS, DS Lite, DSi
19) Wonderswan, Wonderswan Color, not sure what else.
20) NeoGeo Pocket, NeoGeo Pocket Color

^ Obviously not a remotely complete or comprehensive list.