Whatever skills you have are probably skills you could use to help.

If you can draw, see if a project you like needs help with artwork.

If you know multiple languages, see if they need help translating.

If you are good at breaking things, see if they need help testing.

If you are good at being social, see if you can help answer questions in their support channels.

If you don’t know what else to do, just share links all over the internet telling how good their project is.

Everyone can help in their own way. :)