I found my fantastic double room in Rome (89 Euros/night including a huge breakfast) through www.boo.com and www.venere.com.

Agriturismos through www.slowtrav.com, and a Google search.

And like everyone, I double check (and post) reviews on TripAdvisor.

For london, I use a lot of www.londontown.com, for other area in UK outside London, www.smoothhound.co.uk sometime offer better deals than direct from hotels/ b&b; for other area in Europe, I have tried www.venere.com, www,laterooms.com, www.lastminute.com, or www.expedia.ca, the rate might be lots of difference even for same hotel.