At the heart of my music set up is my Mac Pro with 18GB of RAM and loaded with cool software.
For my audio interface, I use a MOTU Ultralite Mk3 and I send the output to a set of JBL 4328Ps with a JBL 4312SP subwoofer. I recommend this speaker system strongly since it is has very accurate response and an included room correction system.
With increasing parity between computer systems and audio equipment, software has become the real distinguishing factor from one studio environment to another. I rely on a variety of DAWs, Plug-ins, Notation, and utility software.
Digital Audio Workstations
My go-to application for recording audio is always Garageband. It is simple, yet powerful, and doesn't ever get in my way. When I need more power, I use Logic Pro. When I have to collaborate with my Windows-based friends (we all have them) I use the Reaper. Finally, I love Ableton Live's different kind of approach to audio as well as to music production and live performance.
Garageband
Logic Pro
Reaper
Ableton Live
Virtual Instruments
In addition to everything that comes with the DAWs, I also have all the Apple JamPacks, SampleMoog, Sonik Synth, SampleTron, Sample Tank 2, EZDrummer, Superior Drummer 2.0, and a bunch of the ToonTrack Drum Packs. I do have AmpliTube, Guitar Rig, etc. but I don't find much use for virtual guitar amps.
Transcription Software
I use Sibelius 7 and Guitar Pro 6 to produce notation and guitar tab. In order to assist my ear I'll also use Vocal Lab and Transcribe software.
Programming For Music Applications
While I started out using Director for MIDI Applications, and I've since moved on to tools like Max6 and even straight Objective-C in XCode for developing Mac and iOS MIDI applications like Guitar Games. CSound is a deep music synthesis environment that I am learning, and will probably always be in a continuous state of learning from it.