Winfingerprint is a Win32 Host/Network Enumeration Scanner. Winfingerprint is capable of performing SMB, TCP, UDP, ICMP, RPC, and SNMP scans.
Platform: Windows
Xprobe is an alternative to some tools which are heavily dependent upon the usage of the TCP protocol for remote active operating system fingerprinting.
Platform: *NIX / Linux
Nmap ("Network Mapper") is a free open source utility for network exploration or security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) they are offering, what operating system (and OS version) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics.
Platform: Windows / *NIX / Linux