(updated May 2017)
If the official Galacticraft download site is down, please use these links to download stable versions of Galacticraft for Minecraft 1.7.10, 1.8.9, 1.10.2 or 1.11.2.
Download the zip file, and then extract all three files from it (GalacticraftCore, Galacticraft-Planets and MicdoodleCore) and place them into your /mods folder. More help on installation.
For Minecraft 1.7.10:
For Minecraft 1.8.9:
For Minecraft 1.10.2:
For Minecraft 1.11.2:
Newer versions may be available via the normal download site.