The slimes that haunt the overworld are from the
Slime Compendium mod.  They are annoying, but they
can provide you with a lot of handy resources.

There are two machines you can build easily:
AAA/BBB/CCC (iron, glass, cobble) = Slime Refiner
AAA/BBB/CCC (iron, refined slime, cobble) = Slime Solidifier

Add slimes and fuel to the Slime Refiner to get refined slimes.

Add refined slime and fuel to the Slime Solidifier to get
raw materials out of the slime.  Most slimes produce Grade-C
Refined Slime which will give you regular slime balls, coal, 
seeds and gold nuggets.

There is also a mysterious slime mutator ...