My 3 favorites:

- Virtual Windows XP AKA 'XP Mode' (not all SKU's)
This is basically a small Virtual PC within the Windows 7 OS that allows you to run the application in it's own contained environment, fully mimicking Windows XP while seemingly sitting on the desktop.
Should make compatability issues of legacy applications during migration a simple thing to resolve.  Think of it as RDP'ing to a second XP box under your control.

- In-box network tracing
Just run netsh trace start capture=yes to start it and netsh trace stop to stop it, allows you to even make it persistent so you can monitor network traffic during the initial startup phase.  No more fiddling around with hubs and promoscuous mode sniffers or port replication.

- Plug & Play smartcards and smartcard readers
Just plug your reader in and insert your smartcard, the P&P architecture should autodetect it and attempt to download drivers from Windows Update.  No need to pre-install the card software or reader drivers (well, as long as the drivers are published).