Kayak.com is very helpful for an easy and quick way to search many sites at once. Their site is simple yet has all of the functions and options to create a search according to your needs. I really enjoy their buzz page which lets you search the cheapest, most recent fares that people have booked for certain cities or areas.

The only issue I dislike about Kayak.com is that sometimes I've been able to find a cheaper fare on an airlines direct website so apparently they aren't always able to find the cheapest fare out there anywhere.
Otherwise it is a great source for knowing what is available for...