If you google ‘netscan’, it will usually be the first or second hit (http://www.softperfect.com/products/networkscanner/).  In the past I've mostly used LANguard and LANspy as my subnet-scanning utils, but lately I've been using SoftPerfect more.  It is free, lightweight (around 700k), and is a stand-alone EXE, so no install needed.  It’s easy to quickly download to a client’s server in a pinch.  It will auto-detect the local subnet and has lots of useful scanning options.  You can see IP, MAC, OS-version, logged-on user, domain, SNMP, open ports, shares, etc.  Two of it’s slicker features are right-clicking a 'found' system to remotely shutdown/reboot (can send a broadcast message first) and right-clicking a file-share to explore the share and/or map a drive to it.  You can pick current or custom credentials to scan under.  It is multi-threaded and scans pretty fast.  Lastly, you can tell by the changelog that it gets updated often (already updated for Win7/Server08r2).