Cell to Singularity - Evolution Never Ends on Steam (steampowered.com)
An evolution simulation.
It has cells, jellyfish, dinosaurs, bunnies and more. It has a classical music soundtrack.
Very moreish. It's one of the nicer free games on steam IMO
Warning - the number of mouse clicks will either strengthen or wear out your Click Finger. Or your mouse.
UPDATE - I found that keyboard clicks work the same as the Left mouse button, so if you have the flush chiclet type key with minimal travel, just stroking the keyboard with your hand pressing lightly racks up the clicks big time. Much less tiring, and it avoids repetitive strain injuries.
Edited