We've been using ServiceDesk Plus for 2 years with great success. It does agentless scanning of our network (given that WMI and some firewall ports are open) for asset inventory. It also integrates with Active Directory. Keep in mind that it's not free and it's not just asset inventory. It handles helpdesk tickets, has a knowledgebase, manages PO's & contracts as well as doing asset inventory. Very nice reporting features, too. However, you can purchase just Asset Explorer for a significantly cheaper price.
http://manageengine.adventnet.com/products/asset-explorer/index.htmlWe also use OCS Inventory in conjunction with SD+. It's free, but requires a local installed agent.
DB