OT: Software Documentation

I work for a small consulting company that creates software for distribution. We're a low volume operation, and don't have many employees. I've been trying to rein in the whole development process to streamline it over the last couple of years, and now I need to streamline the methods by which we create documentation for our products. I'd love to know what systems people on this list use for creating software documentation (manuals, etc.), and I'd love to hear any recommendations for systems that allow for concurrent access (kinda like CVS), change approval, revision control, etc, etc. Are there any good open-source solutions available? I've been searching and gathering information, and I'd really appreciate hearing the experiences of others on this list.