Disclaimer: Installing FU is a one-way trip. FU will affect your game universe and ALL characters. Be sure to back up your universe before installing (Steam\SteamApps\common\Starbound\storage).
Perhaps the largest of the mods for Starbound, FrackinUniverse combines new biomes, a hugely increased crafting system, science, monsters, tiles and hundreds of other assets to the game. It's 3 years in now, and quite encompassing. A few things to expect:
Explore dozens of new biomes
Create hundreds of new armors, weapons and tools
New monsters await
hundreds of new art assets
new quest hub
hundreds of microdungeons
a massive sprawling eldritch dungeon
affect the world around you by changing tile composition through liquids. Fire rain will scorch the ground, and healing liquid will turn sticky slime into a useful healing agent. Hundreds of possible interactions exist.
power your lab with technological creations designed to make your resource-gathering easier and more efficient. Just be sure to have some generators and batteries to keep them going!