Great Western is offering £5 off tickets costing more than £6 to the first 200,000 people that register on its site. The discount will work on any journey you can book through the Great Western site, it doesn't have to be one of it's own routes.
Simply enter your details to register and you will be sent a pin code via email which will allow you access to the £5 off which is per transaction not per ticket. You must register and book your tickets before the 30 November however you can travel after this time. Read the full terms and condition.
Routes included: Bath, Brighton, Bristol, Oxford, London, Cardiff, Exeter, Guildford, Newquay, Penzance, Portsmouth, Taunton, Worcester.
How to book: Visit the special Great Western promotional page to register to get your £5 off pin code. Enter the special pin code when you book your ticket.
I registered yesterday and used my voucher today so this is still working