Anyone have any cool suggestions? My favorite is definitely micropolis (basically sim city classic), I also wonder why GOG doesn't (Yet) sell Sim City Classic?

OpenRA

FreeOrion

Ah I have also installed OpenXCom and FreeCiv, so those aswell I guess

Very glad I made this topic, I completely forgot about freeciv.

2nd place favourite would be Ur-Quan Masters

Birth of the Empires. Not just because I ported it to Java to make it work on Android, but also because it's one of the deepest 4x games. Honorable second is OpenTTD.

My favorites are:

<span class="bold">The Battle for Wesnoth</span> - a very good turn based strategy game with a lot of campaigns, skirmish mode and multiplayer
<span class="bold">Simutrans</span> - a transport simulator with a very good simulation engine

and the already mentioned FreeCiv and FreeOrion.

A few already mentioned. Those that I've had a blast with, along with some personal highlights:

openrct2 - you can choose which funding model to use for each park (entrance fee || ride fees) and the game actually runs (original game doesn't like my gpu)
openttd - there are some great mods to improve animations (eg planes landing and taking off) + add extra vehicles, and
orders are waaaay easier to work with than I can remember in the original (replacing or creating new vehicles with existing orders is almost effortless)
openxcom - weapon selections are remembered for each soldier (thank god), radar distances for bases and aircraft shown on the globe, a setting that makes explosions 3 dimensional with damage along the z-axis

And of course, the higher widescreen resolutions supported for each is incredible, especially in openxcom.

Cataclysm: Dark Days Ahead - driving an ascii schoolbus and crushing ascii zombies (among other things) is weirdly stupendous

Warzone 2100 - gradually expanding your main base

And lastly because it looks really cool (haven't actually tried it) : 0 A.D.

UFO: Alien Invasion

Though I haven't played it in ages.

It's a promising RTS game, plays somewhat similar to Age of Empires and is already well playable, but not as finished as some of the other mentioned games.

The development seems to have hit a few rough patches but I hope they continue to pull through. It looks a lot like the AoE2 sequel I always wanted.

OpenRails??

That might be it indeed, thank you!