Possibly my favorite entry in the Castlevania franchise, Aria of Sorrow just hit US eShop today for $7.99.
For anyone who considers themselves a Iga-vania (Metroidvania) fan and has yet to play this title, please do yourself a favor and download it now. Aside from it's graphical fidelity, I'm of the opinion that it is better than the sequel, Dawn of Sorrow, for the DS.
You play as Soma Cruz, a potential vessel for the Dark Lord himself -- but with this brings the best combat system in the franchise: the "tactical soul" system.
Aside from leveling up, every enemy in the game has the potential to drop their soul, in which Soma can inherit, allowing him to use the abilities of his enemies. There are three different types of souls: attack (Bullet), support (Guardian), passive (Enchanted).
With these combinations, including the large amount of weapons and armor, gives you the freedom to create some really interesting builds. This, coupled with the fantastic level design and rad music, make this a must play for Castlevania/Metroid/Good-game fans.