Open Audit is an Open Source application distributed under the Creative Commons license to provide a hardware and software inventory of systems on your network. It uses Windows VBScript and Linux BASH scripts on the client to post audit info to a PHP/MySql server application.
The application comes with a number of built-in queries and reports for examining the audit data and is easily extensible if you don’t mind rolling your own code.
What I like is the fact that it doesn’t need a client agent installing on your systems, in fact Windows systems can be audited remotely.
I have to declare an interest as I am a contributor to the project, but if you’re looking for a budget auditing solution you might want to consider it.
The project website can be found here http://www.open-audit.org/ and hosts well supported forums if you run into problems. If you do choose to try it out, I highly recommend obtaining the latest code from the Subversion repository.