WINE - open-source WINE allows many Windows apps to run under Linux.
CodeWeavers - sells CrossOver Office, a version of WINE tailored to run MS Office (also runs other Windows apps)
TransGaming - sells WineX, a version of WINE tailored to run games (including DirectX support)
Win4Lin - allows a copy of MS Windows to run as a task under Linux
VMware - commercial product that allows multiple operating systems to run simultaneously on one PC
VirtualBox - open-source product that allows multiple operating systems to run simultaneously on one PC
Linux for Microsoft Windows users - Various articles on using a mixed Linux/Windows environment.
Linux Equivalents - A table of equivalent Win32-Linux software
IDA Open Source Migration Guidelines