The most obvious alternative is OSI (Open Systems Interconnect), a large and unwieldy set of protocols used almost exclusively by military and telecoms companies. It is a seven-layer protocol suite, going right up to the presentation and application layers (unlike the four layers of TCP/IP). Instead of IP addresses you have NSAPs: Network Service Access Point addresses which can be various lengths.
OSI is conceptually similar to TCP/IP up to and including layer 4, and also supports the notion of connection-oriented and connectionless network protocols. Indeed, well-written applications can be capable of using TCP/IP and OSI interchangably.
You would be hard pushed to find a supported OSI protocol stack to replace TCP/IP, but they have been known to exist and put to commercial use.
For more information, Cisco has a useful mini-introduction.
OSI is conceptually similar to TCP/IP up to and including layer 4, and also supports the notion of connection-oriented and connectionless network protocols. Indeed, well-written applications can be capable of using TCP/IP and OSI interchangably.
You would be hard pushed to find a supported OSI protocol stack to replace TCP/IP, but they have been known to exist and put to commercial use.
For more information, Cisco has a useful mini-introduction.