Residenza Cannaregio (Venice, Italy) is available for only 20 Euros/night all-in on May 20th, 2009 on a website specializing in late stays (.com). I've never stayed there, but it appears to be a very nice hotel on the island.

It appears to be a mistake: the hotel is usually about 144E, but is listed at 14E (plus fees).

I'll be curious to know whether anyone uses it (and whether the hotel honors the rate, as I assume they would since you're paying the latestays website, not the hotel).