Which MACs can be converted into a secure unkeyed hash function?
I have ask as part of my answer, "What problem are you trying to solve?"

Do you want a secure unkeyed hash function? If you do, then there are plenty of them around. Even some of the ones that are broken for some uses might be okay for yours (SHA-1 springs to mind -- note the discussion above on HMAC and how broken a hash function has to be). But really, you ought to look at SHA-256, SHA-512, or the new SHA-512/t construction.

If you don't like that, look at any of the SHA-3 finalists. (Full disclosure, I'm the co-author of Skein.)

You're right that HMAC is one of them -- but that's basically the null statement. HMAC is a wrapper around an unkeyed hash function that strengthens it against a number of attacks (and adds a key). If you remove the key (or make it a constant), then all you have is a function with two iterations of a hash function with a bunch of frosting and sprinkles over it. It is obvious that if you start with a secure hash function, make a secure MAC and then remove the key then that you will end up with a secure hash function.

What are you really asking? What do you really want to do?

Jon