Minor revisions (like 2.1.9 (0) or (1) or (2) or (3)) are usually bug fixes. Details can be found at github commit history:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/commits/master
For example, minor revision (3) has the following changes: fix bug with program moveup api; increased ether buffer size for OS 2.3 and 3.0; improve NTP; fix bug in server_change_station that causes buffer overflow when there are a large number of zones.
The latest firmware is the default firmware file you see at the download folder. Previous firmwares are in the archived subfolder.