Airlines & such
RYANAIR - Low costs air line, it is as they say. But be prepared, and read everything in fine print. Even Terms & Conditions. You can find yourself a bargain (considering you are OK with traveling only with backpack, can plan vacations few months forward also, wait 8 to 24 hours for switching planes).
SKYSCANNER This is great site to search cheap flights database. It has few drawbacks - like not refreshing database automatically - it means you need to recheck given prices, and not always rely on given results. In fact, I do have custom JavaScript crawler made for RYANAIR's web site, but it still has few bugs and is fast as average snail.
Sleeping
HOSTELWORLD is not a bad place to find shelter. Found few awesome hostels here. Really.
Couchsurfing If you haven't tried it yet, give it a chance! I can say that couchsurfing changed my travel experience and now I can hardly think about any travel that would not involve couchsurfers.
Other useful stuff
GOOGLE MAPS is THE big thing. I can plan virtual route very easy, one time I could even check how the doors look and what button to press when I will arrive to the hostel. It has so much information, that sometimes it's really a headache picking places you want to go.
MICHELIN MAPS Or route planner - it's even better than google maps, when it comes to route planning. Test it, it's free and really accurate.
WIKITRAVEL WIKIPEDIA and WIKITRAVEL are the pages I think everyone knows about. Or at least they should.
