studentuniverse.com can have flights that are cheaper than buying from other travel sites. you just need a .edu email address to register.
I always found studentuniverse more expensive (or the exact same price) than going to the actual airline website. they also are more expensive than southwest... if southwest flies there.
Cheap rides = 1. Southwest 2. Megabus
Cheap places = 1. Hostels (watch out for bed bugs!) 2. Student host 3. Contact local co-op OR if you were greek, local greek house to see if they have open rooms 4. Couchsurfing.com (if you aren't scared of people -- also only go to a person that has been reviewed IMO)
edit: Kinda got distracted and forgot the question. I never use agencies... find them more expensive. Better to do your research on your own