Devil Daggers is an awesome horde-mode style game that not many people have heard of. Utilizing an aesthetic similar to Quake, the game basically tasks you with staying alive for 10 minutes to achieve a high score. Most people don't last more than a few minutes and fewer, still, have actually managed to break the 10-minute mark.
It is featured in the new Humble Jumbo Bundle 7 at the choose-your-own-price mark. If anyone passes up on this for $1, they're insane. RollerCoaster Tycoon 2 is also in the cheapest tier, along with Runestone Keeper.
The current beat-the-average tier (at time of writing) is about $4.51 and it features Prison Architect, Punch Club, and Stronghold Crusader 2. Paying $9.99 or more will net you Miscreated, an Early Access game on Steam that, honestly, doesn't look very good.
Still, buy Devil Daggers. People clamoring for old-school style shooters are being handed the best deal possible. That it also includes another nostalgia fest with RollerCoaster Tycoon is icing on the cake.