1) Cave Story- A port of Cave Story +, but lacking the new soundtrack and updated graphics. All the Cave Story + extra features and modes are there, plus an added 3D layering effect like the 3D Classics titles. If you've played this game before, having a portable copy with optional 3D can be a really nice bonus. If not, you're in for a serious treat. I'm on my first playthrough right now, and logged 11 hours (almost at the finale right now). It's challenging, fun, and plays like a love-letter to every great platformer, jump'n'shoot, and metroidvania title from the early 90s.
2) Mutant Mudds- This game provides a retro jump'n'shoot platforming experience with "12-bit" graphics, great 3D, and an amazing soundtrack. Challenging and provides great replay value for 100% completionists. Plus, a free update with twice the levels is coming this month!
3) Fractured Soul- Yet another jump'n'shoot platformer, but this one is a bit different... You play on two separate dimensions/screens as the same character, and constantly have to switch screens on the fly to get past obstacles that differ between the two screens, despite having the same layout. The physics often vary between screens, too. Very fun and challenging.
4) Mighty Switch Force- A great action/puzzle platformer. Not much else to say. you run around jumping and shooting, trying to retrieve escaped convicts and solving action and switch based puzzles to do so, all while dealing with enemies.
5) Sakura Samurai- A great 3D swordplay/RPG action game that's designed to discourage button mashing. Requires precision timing that so many action games lack.