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OpenRA is free and created by various people in their free time.
If you want to donate some money towards their effort, you can always post "bounties" on certain issues. The developer fixing the issue can claim the bounty then. On this Bountysource site you can browse for issues to post bounties on.
Public Servers by Baxxster
You can also donate some money to Baxxster who is hosting various servers for OpenRA.
For further information visit this thread on the OpenRA forum.
Own Dedicated Servers
If you don't want to directly donate money, you can also set your own Dedicated Server up to support the community.