Thrive is a free and open source evolution sim, a bit like the early stages of Spore and there's a new release out. With Thrive 0.5.5 their focus has been on improving everything that's already there with big bug fixes. So this means no big new features but it does make a better and more interesting game to play through.
You will see there are some fresh graphical upgrades for cells, long-awaited bug fixes for the cell collecting feature (binding agents) to make it smoother, rival species are going to be much more capable and adaptable to compete with you along with a reworked spawn system. That's really just scratching the surface and you can see their blog post for the in-depth info.
Check out their release video: