Windows 7 Upgrade Advisor: If you're planning to upgrade existing systems that are running XP or Vista, the Advisor can help to identify incompatible hardware or software. It's no substitute for real-life testing but it's a good place to start.
Windows Licensing Fact Sheet: Did you know that Microsoft's volume license programs can only be used for upgrades and not for installation on a bare PC with no version of Windows installed? Did you know volume licenses cannot be used to upgrade any Home versions of Windows? This sheet outlines what's "legal" and what isn't.
Deploying Windows 7 Essential Guidance: This document really does live up to its name, and it contains a great discussion of all the tools and procedures that you'll need in migrating a company to Windows 7.
Windows Automated Installation Kit (AIK) for Windows 7: This ISO file contains the tools needed to install Windows 7 and migrate from earlier versions. The AIK documentation is a separate download if you want to take a look at it first.
Windows Easy Transfer: Although there is no in-place upgrade path from XP to Windows 7 on a PC, this tool provides a way to move system or application settings and data. The other option for this is the User State Migration Tool, which is part of the Windows 7 AIK.
Hardware-assisted virtualization (HAV) detection tool: Not all PCs have the hardware virtualization features required to run XP Mode in Windows 7. This tool detects whether a processor has HAV and whether it is turned on in the BIOS.