You have a monitoring tool that presumably tells you what's on the system. What do you have that tell's you what is supposed to be on the system? The point here is that the CMDB not only tells you what is supposed to be there and how it's supposed to be put together, but also can tell you how you got to the current configuration. If your organization is not interested in at least one of those three, then I would say you should forego a CMDB.
Regarding your comment about "generalizing the stuff," that is part of what Wessel is saying. Figure out your general process (to your satisfactory level of detail) and then evaluate the tools to determine the best one for your organization.
To go back to your original question, you can start by doing what JPTownsend suggested. Alternatively, you can go to the CMCrossroads.com/forums page, scroll down to the CM tools section and look up a few of the repositories listed there, such as Dimensions, MKS, Accurev, etc.
Assuming you are referring to a CMDB in the ITIL context, from what I've read about ITIL v3, the CMDB can be a set of smaller CMDBs or one large CMDB to cover the enterprise. The method you choose to set it up will have a great deal of impact on which tool would best suit your needs. The application has to be robust enough to accommodate the intended contents.