Sometimes all the blocks are posted, and you can join at any point, even after it's all over, if all the blocks are available. Sometimes past blocks are removed...no fun!
Now, I'm talking about BOMs on blogs and websites, not on this forum. They can be started whenever a blogger chooses to start one, so just look around.
Here's a link to some that someone collected:
http://www.quiltingboard.com/links-r...h-t178356.html