Madworld, a mental looking beat 'em up / game show hi-scoring title from Sega isn't far off now. Previews all say it's going to rule:
http://www.eurogamer.net/videos/the-eurogamer-tv-show-madworld

I just got World of Goo, and that is very good too. A bit like Lemmings in terms of cleverness, but great when played with a friend or two.
http://www.eurogamer.net/videos/world-of-goo_gameplay-trailer

And this little quirky title - Let's Tap - sounds great fun, like Vib Ribbon or Parappa the Rapper in terms of curiosity mixed with gameplay potential too:
http://www.eurogamer.net/articles/lets-tap-review

There is also Sin & Punishment 2 upcoming (which should be good if its anything like the mentalness of the original title), No More Heroes 2 which, if it can be as great as the second half of that game was, could prove to be a belter, Wii Sports Resort (a new Wave Race it'd seem) and some other kooky stuff, like a new Punch-Out title, the intriguing looking House of the dead: Overkill and a super duper fast version of Street Fighter 2. Rumours abound too that the Ghostbusters, Indy and TMNT titles will be pretty decent too, but I won't hold my breath there.

Plus the Gamecube back catalogue is being updated to work with the Wii Remote & nunchuk, meaning I'll finally get to play Metroid Prime, Resident Evil 0, MGS: Twin Snakes etc

The Wii still needs a killer title for the year with the kind of development team and time of a Zelda / Mario title, but at least it seems like a few third party developers are starting to realise the systems potential...