Look for deals by manipulating the price sliders for where you want to go. You can use this sign-up link to get $25 off your first AirBNB booking. My own listings are here in Baltimore: the Vivaldi Suite and the Shire.
Worldwide directory.

Budget Travel
Geographically indexed contact information for travelers who don’t want to pay 5-star prices.

30,000 properties, in 180 countries.

Hostelling International
Search among 4,000 hostels worldwide. The Internet Guide to International Hostelling
Among the largest directories of hostels in the world, with 6,000, also message boards and travel links.

You can’t top cottages and bed and breakfasts as the best places to stay in Ireland and Britain — and this site lists them by locality. Thanks to our English pal James Padfield for this tip!