cjrgreen: Although copyright comes into existence as soon as your work is fixed in a medium, nobody will enforce a copyright for you. You get to do that yourself, and you will quickly find that
unless your work actually has significant commercial value, the game is not worth the candle. jamyskis: Pretty much this. Some countries (namely the US and UK) have a copyright office that allows you to register the copyright of your work in that country only. I'm not sure how much it costs there - I think around $30-50 in the US depending on the work concerned.
While this gives you an easier way through court processes in the US and the UK, not registering makes it almost impossible to enforce as you'll have a hard time proving that you own the copyright if the other party falsely claims the same.
Germany and Australia do it a little differently. They have no copyright registration service, and if you want a formal and legally admissible record of your copyright, you'll have to take it to a notary, which costs a lot.
Irrespective of the perceived quality of your work, the point is, if you don't register your work before posting your stuff online, you're going to have a really tough time enforcing it.
Not registering in the US makes it impossible to enforce. But, registration is just a formality. You have to register before taking legal action, there is no requirement that you register before the infringement occurs. And by the time you're about to go to court, the fee and the process of registering shouldn't be an issue.
For some types of works you can register with the appropriate guild. I know the screenwriter's guild has a service for screenplays where they register it and keep a copy for a decade afterwards. Quite useful before you go around showing the script.
But in those cases you generally just need to show that they had access to the work, not necessarily that they used it as that would be impossible in many cases.