I love SpiceWorks 4.0 beta. It is first of all FREE and second of all it can do anything that you want it to. http://www.spiceworks.com/ It does software and network monitoring. It can alert you to ANY change on the network like a user installed a new piece of software or a new user account was created on your domain controller.
It does Exchange monitoring and has extensive reporting. But wait it gets better (damn I feel like I am selling something on late night tv!) it is fully extendable via user created addons and reports.
500,000 Network administrators can’t be wrong right?
You may want to look at Manage Engine’s product suite http://www.manageengine.com/ which is low cost and it works well. You can always look at the free software out there which are ad-based (Spiceworks www.spiceworks.com) and open-source (Groundworks http://www.groundworkopensource.com/).