Megabus is a double-decker bus which now provides long-distance service between 4 points in the San Francisco Bay Area and Los Angeles starting at $1.00 each way - yes, you read that right!  A whopping $1.00 one-way (plus $0.50 reservation fee); the first 4 seats of each itinerary are offered at this rate; thereafter it increases incrementally to $8, $10, $12, $15, $20 up to a max. of $40 each way, which is still a bargain!!
For those of you living in Los Angeles, it gets even better - from there you can also go to Las Vegas, Phoenix and Tempe, AZ at the same ridiculously low rates.  I have two young adults living in the L.A. area that come up to the Bay Area frequently, and rather than flying, this will the new mode of transportation for them now; a lot more relaxing, and saves us a ton of money!!
This service also exists in the Chicago area - check it out.  I don't know how they stay in business:)