Well, the .NET framework is free, recommended. .NET 4.0 supports the System.IO.MemoryMappedFiles namespace classes. Shared memory is a fairly drastic impedance mismatch with the notion of a garbage collector, that's why it took a while. Unless you use pointers, copying from the GC heap to the MMF view is inevitable.
The other IPC mechanisms use shared memory too, it just isn't explicit since its built into the kernel. This all runs at roughly the same speed, a microsecond to setup the mapping and then just the bus bandwidth to do the memory-to-memory copy. Five gigabytes per second is the slowest you'll run into.