Rome2Rio – Not a booking site but great for checking available options and routes, Rome 2Rio will check times and busses door to door. If not sure of your route options, start here.
Skyscanner – budget airline searcher
GoEuro – European travel searcher – all travel modes and options
Skypicker – great if you want to check cheapest options from UK to anywhere – and are flexible
Adioso – flight checker, good if you are flexible with dates
Kayak – airline searcher, long haul and other…
Cheep Travel – booking agency – flights, hotel or both
Spice Jet – internal flights – India
Advice sites: best seat etc…
Which Budget – who flys where?
Budget Airline Guide – a guide to the airlines themselves, where based and links etc.
Seat Guru – check out airline configurations and seat guides
Check My Flight – get details of trips already booked
iFly – airport information, useful if you have a long stop over, or need to change terminals
Flight Radar 24 – live flight tracking overlaid on map