Most forum software allows guest posting, if you find it hidden in the settings somewhere. This would just generate a LOT of spam though, unless you had a CAPCHA on the new post page - but that would be more annoying than having to register.
Anyway, how is having to register, annoying?
I get terribly annoyed when I have to register just to READ a forum. Irritates me to no end. I understand the logistics though of registering when you want to POST.
You'll get a lot of spam and imposters when you have a forum without registration. If anyone really has a purpose to post, then registration is no problem.
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You can allow guest posting, but this I think would hurt the "community" feel of things, as you will never be sure who it is that is actually posting under a certain name.