IPSim is a network simulator, used in the University of Salford to help teach and assess Computer Science (and related courses) students on their subnetting and static routing skills. It is much simpler to use than Cisco Packet Tracer, and so is more suitable for people learning about networking but without the intention of becoming a network administrator.
IPSim and the required JRE are included in version 1.2 of the Live CD that is distributed to module students.
You can download IPSim version 1.4 from here - this needs you to have a JRE (v1.5 or greater) installed. If you haven't then please use the Live CD.
Version 1.4 is stable enough, and bug-free enough, to be able to be used confidently for student assessment. It does occasionally catch an exception, but so far nothing serious has been found.
There is a newer version 1.6 available, which gives better feedback on some of the "good practice" issues that can mean 100% connectivity doesn't get you full marks in the practical test. It will also add random faults to a working network, to allow you to try to fix them. However, this contains known bugs and instability, and so it is only to be used by people who don't mind that sort of thing.
Please note I no longer have the source code for either version, and so there is no possibility that bugs will be fixed or new features introduced.