Donating

OpenRA is free and created by various people in their free time.

Bountysource

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 Sleipnir's Stuff.

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.