I just returned from a vacation in Europe, and I found two Mapplets invaluable for planning my trip:

Wikimapia Layer --Wikimapia.org is a website where people have have annotated millions of places all over the world, and you can now view these annotations on Google Maps using their Mapplet. After zooming in on a location such as [Prague], I would then go to the "My Maps" tab and turn on the Wikimapia Layer to find out where interesting sights such as the Old Town, Prague Castle, and the Charles Bridge were located.

Booking.com Hotel Search -- Booking.com is an online hotel reservation service that has especially good coverage in Europe. Using the Booking.com Mapplet, I was able to find out which hotels were available and compare their rates and proximity to the locations I was most interested in seeing. Using this, I found the Hotel Modrá Růže, which I highly recommend if you're ever visiting Prague.

Note: If you're traveling within the US, you can also try the Hotels.com or Orbitz Hotel Search Mapplets.