^ Yeah I was going to mention Battleships Forever, amazing game! Doesn't matter that its ten years old now, the style is still cool, the gameplay is still amazing the concept is still simple and unique and the price is still free!
The game may not be exactly the same as Cosmoteer, but BF has a lot of strategic depth and is very focused on the RTS side of things.
The ship design is more limited, but overall it's still a ton of fun, and more games should be copying it.
http://www.wyrdysm.com/games.php
Starship Theory is a new release (literally weeks ago), still early alpha/beta, I'm waiting for it to get a bit more developed before diving in.
Reassembly is ok, a fun ship building game, but felt like it needed more.
Boss Constructor, is another ok game has some bugs and issues, didn't really blow me away like Cosmoteer
Captain Forever Remix, is a fun and simple ship builder, don't expect it to keep your attention beyond a dozen hours at best, but it has some nice ship building nice combat, and also looks quite similar too Cosmoteer
Defect is a great game!
I rate this much higher than the Steam reviews. It's well made, well rounded and has some of the best looking ships in any shipbuilder. Defect is purely single player, but as well as a sandbox mode, there is a decent campiagn with a variety of interesting missions that will stretch your ship design skills, and your sense of humour.
Defect is very funny, but best of all, David Bowie steals your spaceship.
Avorium is ok, but there are a million games like this, all doing the Minecraft thing in space but without creating more interesting/rounded universes to explore. Similar games include Space Engineers, Starmade and Empyrion.
I thought Rimworld was a great comparison. Even if you don't build or control spaceships (except when you escape at the end), the top down crew management involved is incredibly detailed and fun. If Cosmoteer only includes a fraction of the crew management in Rimworld, I will still love it immensely.
Also Rimworld does NOT cost $500! it costs just £23 for the complete game, and its very good value for money
Galactic Princess, is a little gem I've been keeping an eye on for some time now. Its still in development, but apart from side view 2D ships, that you design and alter, there's deep management & RPG systems for the crew, as well as a detailed universe to explore and adventures to go on. Plus incredible pixel art.
https://galactic-princess.com/
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Starship Corporation has a great game concept; Running a company desingning spaceships to order, plus manageing ships running various routes and missions. Sadly though SC's execution is subpar. If you are desperate for a new ship builder and don't mind something buggy and incomplete, then get SC during a steam sale.
I backed SC on kickstarter years ago, and I would still back it again given another chance, because it is a great concept. But, with the best will in the world, the dev just isn't a good enough programmer to pull this off well, and fair play to him, he warned people during the Kickstarter, that he wasn't a programmer just an artist with a good game idea.
FTL is the other obvious comparison. We all know FTL, if not, go and play it now!
I'm sure I've missed a few good games from this list, I might add some more later.