Tetris Party and The Ancients pinball game is excellent. I dont recommend Tetris though if you're going to play it on the 3DS, because the controls with the analog slider are really bad (too touchy) and the placement of the dpad makes using that extremely uncomfortable.
X-Scape hasn't been mentioned yet, and it's a really good game. It would have been worthy of a full release in my opinion. Reminds me of Star Fox a little bit, but it's quite different too, and it's a decent length.
Also, Pictobits (Or Picopict in Europe) is great if you're a fan of classic NES games. The music alone is worth it to be honest.
And another is Ivy the Kiwi? Mini. It's half of a retail release, and has 50 levels - easy enough, unless you get all ten feathers in every level. I've done that, and it gets progressively harder. It's like an iPhone game - you have a score based on time, feathers collected, etc., and it's great for quick bursts of play.
And it's actually a good game. Arguably, it's the type of games that a mobile gaming service like DSiWare should have a lot of.
My favorite part of the day is when she dance in the mirror.