good games for low spec PCs

I dunno. Multiplayer old games can be tricky because the network code mostly sucks.

Command & Conquer has a decent online free port here. The community is crusty as fuck though, so beware.

Soldat is a fun 2D side scrolling multiplayer arena shooter. Don't be dissuaded by the 2D aspect of it because it has a pretty cool physics engine, the soundfx are real punchy, the graphics are gory, and it has all the fun elements you would want in an arena shooter, like deathmatch, team deathmatch, capture the flag, AI bots (that are actually pretty good,) infinite ammunition, and a bunch of other customization options.

The gameplay is pretty straightforward and addictive, it runs at pretty high FPS even on my crummy laptops, the netcode is pretty good, it's actively played today, and comes in a portable ~30MB download.

I don't know open sourceness, but there are level editors you can download so you can make your own maps. I personally didn't have as much fun with the map editors as I do with Doom, but the game itself is very efficient for its filesize and spec requirements, and loads of fun.

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Looks like I didn’t catch the multiplayer part, what a cunt I am! Oh well, if ya got a spare 286 lying around folks, play Civ on it.