last minute hotels

booking.com
LateRooms.com

I've used hotwire.com...but you don't know the name of the hotel until you book (that's why they give good rates). My gambles usually pay off with a Holiday Inn or other chain hotel...

Booking.com
Venere.com

(I only use Hotwire in the US, although you should be able to score a business hotel from it easily, even in Europe)

There's and app called "Hotel Tonight" that offers last minute deals on rooms for that night.

I would use www.hrs.com

I use a combination of travel guides and trip adviser type websites to create a list of places that would be good choices. I don't travel on a strict itinerary so I rarely book more than my first and last night ahead of time (those are set because of my flights). The day before I'll arrive in a new town, I pull out my list and start calling hotels. First I let them know what I need (e.g. a room for two for three nights). If they have something that works, I ask the price. If the price is good, I book it and let them know when I'll arrive.

I've only had a problem once - opening day of Oktoberfest in Munich - but have never ended up homeless. Generally, I pay less than the posted rates because places are sitting on rooms that will otherwise be vacant.

Often the hotel will offer something that wouldn't have been an option booking ahead, like renting an apartment (owned by the hotel) for only three nights rather than the normal week required to book ahead.

You could do something similar. When you are definitely on a flight, call the hotel(s) directly and book your room.

Thanks everyone. Your comments were very helpful!