Apparently Nintendo's WiiWare service is finally offering demos of their games, though only some are available and it seems to be either the whim of Nintendo or of the game publishers that determines which games will and won't have demos available. Nintendo is about 3 years late with this option, but I suppose late is still better than never. There's a lot of quality software on the WiiWare service that I suspect would have sold much better out of the gate if demos had been available (and some, like the subpar port of Tales of Monkey Island, that I would have avoided if a demo had been there to warn me).
Of the demos that have just become available, I highly recommend checking out World of Goo if you somehow haven't played that yet, and Bit.Trip Beat if the description "retro rhythm game" does anything at all for you. The Bit.Trip series is crazy hard, but also really rewarding, fun and unique.
Also unique and carrying my unhesitating recommendation is NyxQuest: Kindred Spirits, a game saddled with a terribly unfortunate name (it was originally Icarian, but apparently that didn't fly with the Big N) but graced with surprisingly deep gameplay. A physics-based platformer not entirely unlike Trine, NyxQuest sees you walking and flying a fragile but nimble winged girl through a treacherous desert, using the IR pointer to shift giant blocks and redirect fireballs at enemies so as to craft a safe path. You're controlling her directly with the nunchuk, and the world with the Wiimote, and it all comes together very nicely - the levels are well designed, the physics feel good, and the game looks and sounds pretty great for a WiiWare title. Now that there's a demo out, if you own a Wii you really have no excuse not to check it out.
Some other WiiWare titles I think you should keep your eye out for a demo of, just in case Nintendo bothers to go back and put them out:
- LostWinds (and its new sequel, Winter of the Melodias)
- Onslaught
- MegaMan 9
- Excitebike: World Rally (actually, if you like Excitebike, just buy this one now)
- Tetris Party
- Toki Tori
- Swords & Soldiers
And there's plenty of titles I'd like a demo of, because I hear they're good but I haven't pulled the trigger on them yet...
- Bomberman Blast
- Art Style: Rotohex, Cubello and Orbient
- Space Invaders Get Even
- Water Warfare
- Contra Rebirth
- Groovin' Blocks
WiiWare is a really good service, and has the potential to become great. World of Goo, NyxQuest and LostWinds are solid examples of how to use the strengths of the Wii control scheme to the benefit of your game - they use the pointer better than many full retail releases - and both those games and the delivery system as a whole deserve more attention than they get. I suspect the number of Wii owners who don't even know what WiiWare is or what's available there is huge, and these demos are a step in the right direction. One that comes surprisingly late, but hopefully one Nintendo will follow through with.