I would recommend going back and having a look at some of those open source efforts. I agree you won't just be able to drop them in place, but you may find that you can reprogram their login system fairly quickly. If nothing else, take one, rip the guts out of it and use the skeleton to model your program on. Open source isn't an all or nothing effort, you are welcome to use the parts of it that work for you, and discard the parts that don't.

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Jeremy
I didn't believe in evil until I dated it.