One of the members on the Exetools, named Terrayoot, has created a plugin for Olly named "Olly Invisible Plugin" with some interesting code which seems to be developing into a very nice plugin indeed. You can check out his work and the informative discussion of debugger detection code in this thread:
It seems to be very effective, but is still under development, so I'm not going to attach a copy of it here yet.
There is also an interesting small piece of code by TQN, who is a member here, called "AntiDBG" which is useful for determing whether your Debugger is well hidden from most current methods of finding it. Since TQN is a member, I will allow him to decide whether or not to post his detector here also.
One of the members on the Exetools, named Terrayoot, has created a plugin for Olly named "Olly Invisible Plugin" with some interesting code which seems to be developing into a very nice plugin indeed. You can check out his work and the informative discussion of debugger detection code in this thread: