OmniMIDI (formerly Keppy's MIDI) is free and Open Source software for Mac OS X. It's a fork of Kode54's BASSMIDI Driver for professional use, which is quite an extensive fork. OmniMIDI has several unique features such as automatic render recovery, support for more than 50 types of MIDI and hardware devices, support for ASIO drivers, support for VST/SAAS/IAX and ALU/VSTX and much more. You can download the latest release from their website. It has been tested on a Dell laptop running MAC OS X Tiger. A current release is v2.5, but there are also future releases scheduled, which could bring some new features and enhancements.
One feature that really excites me is the ability to render my custom synthesizer sounds in real time using an application called Cubase LEAP. In fact, I have two different versions of OmniMIDI LEAP that I use on both my MAC and PCs, and each version is completely free! My one laptop that uses the free version is always on my desktop in front of me while the other laptop is in my bag, so it's literally just inside reach whenever I need to use the midi interface. My point being that wherever I am, OmniMIDI is just a few mouse clicks away. If you work on your synthesizers at home or in the professional world, it would behoove you to get a free version of OmniMIDI.
Cubase LEAP is one of the easiest ways to get a hold of your own virtual OmniMIDI interface, and you can download it free from Cubase's website. Then you will be able to start setting up your own virtual studio right away with ease. OmniMIDI has a ton of potential for anyone looking to create their own sounds, and I think that this software will become super popular. Please check out the website for a lot more information on this wonderful electronic instrument, as well as other interesting applications.