Sunwait is a small C program for calculating sunrise and sunset, as well as civil, nautical, and astronomical twilights.
While developed under Linux, it should work on any system with a C compiler and the C standard library.
With some minor patches provided by George Gugulea, it will compile on Win32, which results in sunwait.exe, but I haven't tested this extensively.
The feature that makes this program slightly unique is that it can be set to wait specific event (such as 5 minutes before sunrise), then exit.
This makes it useful for 'cron' jobs or 'scheduled tasks' when you want something to happen relative to sunrise, sunset, or some other astronomical event.
For example, consider this line from my crontab:
01 00 * * * sunwait civ start 38.794433N, 77.069450W ; br b6 off
This line executes at at one minute after midnight, waits until the start of civil twilight, then runs the command br b6 off (which turns off my carriage light).