There was a thread not too long ago about games that surprised you with awesomeness.  With a few tweaks, my list for that is pretty much the same as overlooked gems.

Art Style: ORBIENT - This is such a simple game, but it was completely addicting.  All you do is play around with gravity.  You play as a star, and you only have two control options, 'A' for attract and 'B' for repel.  You absorb smaller stars to grow in size or catch them in your orbit to score points, all while avoiding crashing into larger objects or getting sucked in by black holes.

de Blob - One of the best 3D platformers I have played not named Mario.  Who knew painting could be so fun?  Great use of visuals and music.  It is very rewarding to turn the completely white environments into a rainbow of colors.

Fluidity - Fun little puzzle platformer.  You play as a puddle of water.  Yes, a puddle of water.  You use tilt controls to move the water around.  Eventually you get new powers, like turning into a block of ice or becoming a cloud of steam.

LostWinds: Winter of the Melodias - This one is an exploration based platformer.  It makes great use of the Wii remote IR pointer to control a wind spirit that helps the little boy you control with the analog stick.  The first was good, but the sequel felt like a big improvement.

Muramasa: The Demon Blade - I wasn't too sure what to expect of this one.  I heard it was good, so I gave it a try.  It turns out it is just a fantastically well made, high-quality game.  Beautiful visuals.  Amazing boss battles.  Tons of fun.