I admin a small company that has about 50 users. We're able to track all of our assets (workstations, servers, monitors, VoIP phones, etc) using a simple excel spreadsheet. we also use spreadsheets that document the hardware configs for our servers and visio for the network set ups.
the important thing that I've learned is taking the time to maintain the inventory once you've set up how to track it. if everyone in the department doesn't keep up with it...the inventory is useless.