I've been waiting for a year. For those who don't know, this is a complete redesign of Binding of Isaac, a procedurally generated NES-Zelda-ish dungeon crawler. Rebirth has double the number of items, more enemies, more bosses, more room designs, new music, new graphics (pixels art!), and perhaps most importantly, it's not a Flash game, so a bunch of stupid limitations and bugs from the original are long gone. BOI was an amazing game in spite of the technical problems, and I'm hyped out of my mind about Rebirth. This'll probably be my November.
If you own the original BOI, you can pre-order this on Steam in the next two days and get a discount.
http://store.steampowered.com/app/250900/
http://bindingofisaac.com/