One of the things Kristin and I talked about over the weekend regarding our RTW trip was the prospect of not using any air-based transport as we made our way around the world. What if, instead, we crossed oceans on freighters or repositioning cruise ships? Personally, I'm not a big fan of cruises but I think if done with an eye towards keeping things cheap and with the goal of achieving surface-level transport and not "vacation", it could be a way to really gain an appreciation for just how big our planet is. Especially in today's ever-shrinking world. After all, if we quit our jobs and sell/lease our house, we'd theoretically have all the time our budget allows.
A quick glance at Freighterworld.com reveals dozens of possible trips at little more than $100/day per person. Just look at some of these options departing from the east coast of North America to the Mediterranean. There are dozens more that leave from the west coast and circumnavigate the globe. Sure, we'd have to provide our own entertainment and we'd only have 6 to 24 hours in each port (my main complaint with cruises) but if they were ports we wouldn't otherwise even visit at all, then it could be worth it. Especially if, again, we just view it as slow, budget, transport.
There's also cheap last-minute deals to be had on cruises as well. I saw last-minute deals on the QEII yesterday for a 6-day transatlantic cruise from New York to the UK for about the price of a roundtrip plane ticket. Flexibility and a willingness to be spontaneous will certainly come in handy in terms of travelling on the cheap.