Alpha 21 Stable b324 is now out!
The Fun Pimps wanted to make it available in time for Summer Sale and 4th of July weekend to everybody out there!
A21 b317 savegames are compatible with A21 b324.
Official Release Notes
Please find the full Release Notes here.
If you missed the last Experimental build, you can see the changes here.
If you want to report a bug, please follow this link to do so.
TFP wants to extend a big thank you to all their fans for supporting the game and wish you a happy 4th of July!
Source: Official Forum Topic