This is a website I use regularly to find the best fares for a particular trip I am needing to take. As I have a high status with BA I restrict my searches to oneworld Alliance airlines.
The website is great for finding fares either for specific dates, or over a period of up to a month for any length of stay. Those who are in a specific airline alliance or prefer to fly with a specific airline from a specific airport can do that with accurate results. It will provide results from/to any commercial airport worldwide - particularly useful for those looking for the cheapest option to bring a partner/family member from BKK to any UK airport.
Once you have your results you are then able to filter them to find the best flights that suit your criteria.
You cannot book from the site, it merely checks the flights from the airlines, you then contact the airline direct (website/phone) or use your usual travel agent booking site.
The website only provides results for 'published' fares so any special 'bucket' fares will not be shown.
It's saved me thousands over the years - happy hunting!
Matrix Airfare Search