SpeedX 3D - $3.99
It's an endless smartphone-like game, but the 3D makes it very fun to play for hours, and hours, and hours...
Fun! Fun! Minigolf Touch! - $4.99
Minigolf game with lots of stuff to unlock. I didn't like it, but if you like that kind of game then maybe you should give it a try.
Gunman Clive - $1.99
2D side-scrolling adventure. Short, but the levels are simple and nice to replay with different characters.
VVVVVV - $7.99 (not finished yet)
2D Metroidvania-like game. I heard from people to have a long replay value due to hidden gems. Personally I don't know, but the game looks nice enough.
Bit.Trip Saga - $19.99
Music game that covers all of the Bit.Trip games released on Wiiware/PC (except Runner 2). Lots of replay value here.
DSiWare?
10 Seconds Run - $2.00
Title says all. I've spent hours on it!
Metal Torrent - $5.00
Bullet Hell game. Short, but satisfying.
Pyoro (no trailer) - $2.00
Another smartphone-like game.
Uno - $5.00
The rules covered by this game and the decent CPU makes me play it fairly often. Did I mention it has online too?
I covered what I think that are the most re-playable games on the eShop, with the exception of VVVVVV because I'm still playing it.