There are half a dozen or so low-cost inter-European-city airlines. RyanAir is the largest. I recently got a round trip on them from Frankfurt to Pescara, Italy for about $90 -- and this is one of their more expensive destinations. Other flights are ridiculously cheap. If I'd wanted to go to say Pisa or Stockholm from Frankfurt, the one way ticket would be 10-12 euros. London (Stansted) to Rome is 10 euros. These are ultra non-frill flights, and they all offer one-way trips without jacking up prices. One major disadvantage is that the airports can be out of the way. For instance, the Franfurt one is actually 62 miles from the city, but for these rates I'll take a train or bus to the airport.

-- Lloyd Kahn

Ad from RyanAir website advertising limited come-on flights to the above cities for 99 pence ($1.80) one way, starting next year. On many flights the taxes will cost more than the fare.

Ryan Air

In addition to Ryanair, there are others with the same idea, with less extensive routes:
Flybmi
Easy Jet
EuropeByAir
Spanair
Virgin Express