Get more cash, these are my must-haves
X-Scape (FPS with some strange mechanics)- This game is amazing. A total bargain at $8. I don't care if you're one of those weirdos with an anti-DSiWare stigma, GET THIS FUCKING GAME.
Mighty Switch Force (Puzzle Platformer)- Great little game. Not too lengthy but worth $6
Art of Balance: Touch (Puzzler) - Try the demo first. An addictive game with plenty of content for $7
EscapeVektor (Arcade?) - Yeah, I can't describe this game in a few short sentences. Go look up some reviews.
Dillon's Rolling Western (Tower Defense) - Don't touch if you don't like TD games due to repetition. If you do, this game is a blast.
Order Up, Outdoors Unleashed: Africa 3D and Gunman Clive are all pretty decent as well for their genre. Same with the pinball games and Sakura Samurai if you like Punch-Out.
Some other good DSiWare would be Mighty Flip Champs, Escapee Go, Flipper, AlphaBounce, Glow Artisan, Starship Defense, Trajectile, Aura-Aura Climber and Flametail. Not gonna list all of the good ones.
And overrated/warnings
Cave Story - Fuck Nicalis. They're the goddamn Capcom of indies, all they do is port, port, port with minor features added (occasionally REMOVED) at an even faster rate then Capcom. Hell, unlike Capcom, they don't even develop.
But I digress, because beyond that, this is still an amazing Metroidvania game, even after the, I don't know, 7th or 8th port around.
VVVVVV - Again, fuck Nicalis. This one has removed content and costs more then Steam
NightSky - Fuck Nical- No wait, actually, if you haven't gotten it before, then this is the best version due to nothing being changed except 3D and portability.
Denpa Men - I like dungeon crawlers. But if you don't, you may get bored.
Pushmo - OVERRATED AS HELL. Can't really describe anything else, but you got Crashmo and that's supposed to be better.
Mutant Mudds - Generic platformer. The main levels are fun, but start off extremely slow and seriously, it's really generic. The DLC levels are an ATROCITY.
Shantae: Risky's Revenge - Overrated again. It's still above par, but the useless map, annoying backtracking and occasional bouts of boredom don't all work out for the best.