As well as the options already pointed out, there are also a lot of open source systems out there.
There is an open source experimentally minded D-Star network out there called the multi-trust which has its own reflectors based on a system called DExtra. It's similar to DPlus but with a completely open protocol. It includes D-Star repeaters using sound cards connected to radios. Experimentation is encouraged on that network, which is in the true spirit of amateur radio. Jonathan G4KLX
