Yes, Reaper is coming on very strong.
It still has issues to cover but the speed at which it's being developed is very impressive and what it already does is a lot. The author is the genius who created WinAmp.
It is a very light and fast program (I believe it can be run from a pen-drive) which does audio and MIDI and accepts VST and VSTi plugins.
The trial version is fully functional and it technically doesn't expire.
It's very cheap and it already has a large community of users.
But: Reaper is nowhere near Audition in terms of built-in audio capabilities.
It is mostly a sequencer, so either on Mac or PC it will require some audio-dedicated add-on for many tasks.
An other alternative you might consider is buying a piece of hardware that you have a use for (like a Zoom Pedal, a Presonus or Tascam sound card) which comes bundled with Cubase LE or LE4. It's a good software, and being free doesn't hurt. I have LE which seems OK although I'm considering the Logic route (if I can justify the investment). LE4 is more capable but I hear it's less friendly to install, while the other versions of Cubase even require you to use a physical dongle.
MU.LAB is an other option, and it has a free version (6 tracks, 16-bit).
But really, if you don't need to export to MIDI, GarageBand comes free with OSX and could possibly do the job until you decide to comit and pay for a better software (read: Logic Express is not too expensive and has most of the essential features of the full Logic Studio, in version 8, not version 7).