Going to London for a week in Mar 2009. I thought my low budget would not get me anywhere far, but I just got train tickets to spend sometime in Paris for a mere 29.50 GBP one-way! Got it from Eurostar.com. The catch: It is non-refundable and an overnight stay is required. Works for me!
That is pretty good, and the overnight stay is actually better than the same day round trip deals they usually sell. In fact I am convinced that most who bought the same day roundtrip tickets used them as one-way transport, tossing the return ticket.
Beware of other websites that try to appear like Eurostar.com... one had their logo and was offering a roundtrip promotion for US$156.