EthernetMIDI is a free, open source (GPL license) application that allows forwarding MIDI data between 2 PCs using arbitrary network devices that provide fast IP forwarding. A 100Mbit LAN will work fine. For now it runs only on Windows but it will soon run on OS X and Linux too.
News and Roadmap:
We are currently working on an improved user-friendly and cross-platform version. It is also planned to be integrated into the VST and AU plugin version of LinuxSampler, to allow using cheap computers (even without sound or graphic card) as sampler slave machines being used in standard sequencer applications like Cubase, Logic and Reaper. Please support the development of EthernetMIDI and LinuxSampler by donation. Be fair and help us improving those free tools for you!
Please see the Documentation in the README.txt file contained in the ZIP file. EthernetMIDI does yet not come with a Windows installer, so the installation is done by unpacking the ZIP file in some location and then if you want make a link to the EthernetMIDI.exe on your desktop.
EthernetMIDI (for now) requires a virtual MIDI loopback device for Windows. I recommend either Maple or MIDIYoke. Since the site hosting the Maple tools seems currently be down, www.marblesound.com, I've temporarily mirrored it locally. You can download it here: MapleVMCv352.zip
Source code: ethernetmidi-src-0.0.2.zip
To compile the source you need a windows C++ compiler, I used mingw and msys. Get it here: www.mingw.org
and the Qt toolkit, either the commercial version www.trolltech.com or the free GPL version qtwin.sourceforge.net
For any kind of feedback contact me at the email address listed here