Google Maps. You can zoom in, drag the map around, get directions. HOLY CRAP IT IS TEH AWESOME.
The mere existence of Google Maps makes Mapquest, MSN or Yahoo Maps look like they were designed with stone tablets. And, unlike Microsoft’s Streets & Trips, it’s free.
“[Google]… is there anything they can’t do?” — obHomerSimpson
UPDATE: Rafe provides much more articulate praise than I do.