From MoneySavingExpert.com

Southern 90% off Advance tickets
Book online, one day only

On Thurs 26 May only, get 90% off Advance single tickets from Southern Railway. You can buy tickets for up to 12 weeks ahead (14 August 2011). Plus Senior, 16-25, Friends & Family, Disabled or HM Forces Railcards can be used!

Southern Rail services the South East, eg, London, Portsmouth, Brighton, Southampton.

How do I get it?

Buy tickets from Southern Railway on Thurs 26 May. If the route you've chosen has sale tickets available, the price will show as 90% off, eg, London to Brighton will cost 45p.

Tickets must be booked for specific journeys and you must travel on the train you've booked. Buying online, having tickets delivered or collecting at the station is free.

Anything else?

Southern has said around 750,000 tickets are available, but they're still likely to go fast. Be warned, the site may be slow from heavy traffic.

Routes included: London to Brighton, Portsmouth and Eastbourne.
How to book: Visit Southern Railway website
When does it end?: Thurs 26 May 2011 only