I have linux running inside Virtual Box on my Mac, used mostly for ... testing things that are/were supposed to work on linux (most recently, someone was having troubles with avr-gcc on mint. Turned out to be at least partially due to mint putting together a broken avr-gcc package.) Virtual Box works nicely, and is free. I've got VMs for Unbuntu, DSL, W98, WXP, W8 (preview), and mint, at the moment. For windows, the licensing morality is annoying (if I have one VM for running Atmel code, and one for running Microchip code, and one for running Cypress code, and I only run one at a time, all on the same Mac, how many licenses am I supposed to have needed to buy?)