I work from my own broadcast-standard studio, based around Mac Pros running industry standard production platforms Logic 8 and Pro Tools. I also use Digital Performer.
Other computers run samplers including GigaStudio and Kontakt, with orchestral sounds from Vienna Symphonic Library and East West Quantum Leap Symphonic Orchestra. Virtual instruments include Native Instruments' Absynth, Reaktor, Massive, Pro53 & FM8, and Spectrasonics' Atmosphere, Omnisphere and Stylus RMX.
When composing for acoustic instruments I tend to start off with good old-fashioned pencil and paper, before transferring into industry-standard notation software Sibelius for detailed orchestration, preparation of parts etc. I also have a large network of highly experienced professional instrumentalists to call on, with whom I've worked extensively.
When composing and designing sound for theatre, I regularly work with standard show-control software including SFX 5.6 / 6 and QLab. Other performance-related / live media software packages I use include Max/MSP and Isadora.