The original reference implementation of the RTP-MIDI protocol (originally specified in RFC4695 - now updated to RFC6295) from the guys who developed it is available as part of an older version of their sfront software.
It seems that the Wikipedia page on RTP-MIDI now has a good list available implementations.
A more up to date implementation is available in the Java based nmj library, which apparently works with iOS: http://www.humatic.de/htools/nmj
There was (now defunct) the interesting scenic project that contains Python based rtpmidi support (from which the now discontinued midistream Debian package was derived). In the Scenic source tree the actual midistream python application is here scenic/py/scripts/midistream.in, and the associated library may be found here: scenic/py/rtpmidi
It is also possible to use JACK to send MIDI but you need to run JACK on all machines (so for iOS you'll need an app that supports JACK).
Some more Windows based software (free but not open source): http://www.tobias-erichsen.de/software.html
Update 2018: There's now a Node based implementation of rtpMIDI and also the Arduino-AppleMIDI-Library that may be possible to port to Linux.
Update 2019: A new implementation aimed at the Raspberry Pi called RaveloxMIDI has become available which seems like it will run on Debian/Centos Linuxs as well.