Subscribe To Thread might help...
I am not sure, search this forum, you might find something..
I think Subscribe to thread is a good thing, cuz all you need to do is go to your UserCP, and check the Subscribed Topics there.. If your forum is huge, then ya, use that.. I never heard of Post Bookmark tho... You dont really need a post bookmark, cuz you always can subscribe to thread, and if new messages will be posted, i think you get Email (Can be On or Off)... And its easy to access the thread by going UserCP as i said before...