Free-to-play is the way to go for LAN party games. Everyone downloads, everyone updates, no-one pays, everyone wins. However, in the past, people had to choose between free games and good games. Not so anymore. Tons of games now are not only free, but also ridiculously awesome. Here are the best of the best.
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Theres a few old RTS games that are free I think, theyd make good LAN games though some will need custom patches/ Gamerager or a similar service.
I still play old rts games like AoE2 with friends, something about those old RTS games that just feels right compared to some of the newer ones...