"Spiritual successor" to Cave Story, Kero Blaster is Daisuke Amaya's latest game. Although not freeware like Cave Story was (the game is $8, although can be bought in a bundle with the soundtrack for $10), after one playthrough it definitely feels worth it.
The gameplay is very similar to Cave Story, although it definitely has a few key differences in the engine. The level design is spectacular, the graphics are beautiful, and the story is a good kind of quirky, albeit somewhat short (I beat the game in less than 3 hours with no walkthroughs or anything).
Imma stream it sometime today (tomorrow for some people? may 12th), probably around 1 or 2 Mountain Time if anyone wants to watch.
There is also a free, short demo featuring one of the other characters, Pink Hour.