Custom Actions will provide you with a NodeRef of the resource or space that the action is acting on. Once you have it, though, what do you do with it?
This page is the opening index for a list of things that you can do with a NodeRef. If a question looks like 'How do I use a NodeRef to...' then it probably belongs here. Hopefully some kindly developers will stop by and fill in some answers.
Some services are required, you can get them this way: