I've made it no secret over the last few months that I'd lost interest in my Wii despite wanting to love it.  Rejuvination can come in the unlikeliest of places.  I recently purchased Excite Truck due to how much my son loves Mario Kart.  I had completely overlooked this game before.  Turns out I'm having a blast with it and it has now gotten me back into Mario Kart which I had barely touched.  It has also motivated me to finish up my Luigi playthrough in Galaxy and I've also bought Boom Blox which is pretty cool.  I'm actually looking forward to buying more Wii games for the first time in many months.  Can't wait for Punchout!  Excitebots is now also a real possibility.  That is, once I've beaten Excite Truck.

I'd love to hear from people about other games that are underappreciated on the Wii that I or anyone else may have overlooked.

Here's the obligatory: Okami.

I also really loved SSX Blur, but apparently that's a bad thing on this site.

wfz SSX Blur was a masterpiece of sub 37 degree H2O pwnage nothing tobe ashamed of.

Metal Slug Anthology if you like Metal Slug series.
Battalion Wars 2 is pretty cool too.

You can probably pick both off ebay for pretty cheap!

If you haven't picked up World of Goo yet definitely do it!

you should check out this game i played awhile ago.. it was called zelda. i dunno if anyone has really heard of it, but i thought it was pretty cool

Yeah. I like Excite Truck a lot. It was one of my first Wii games. Another one was Super Swing Golf. I played that game a lot, but never got good at it. It was really fun though.

@ OP You had me until Boom Blox.

I think that Dewy's Adventure was a good game and only like 83 people have played it.

Excite Truck might just be my all time favorite racing game. I can't believe the middling reviews it got!

I've been playing it again the last couple of days, too, for the first time in a while. Finally got all the S ratings on all three modes. All that's left are the challenges. Oh, and the monster truck you get for the S's is pure AWESOME.

I have so many great games on the Wii now, I can't even decide what to play half the time.

Oh, and you may want to check out No More Heroes, and Battalion Wars 2. Those didn't get nearly the sales they deserved, but they're both really great games.

If you're the oldschool type: Fire Emblem: Radiant Dawn.
RE4 is obviously a masterpiece.