I've got purple carrots, yellow beetroot, red-veined chard and tri-coloured pak choi from Thompson and Morgan, and stripy tomatoes (bit late for this year, though), white beetroot, purple climbing French beans, yellow dwarf French beans and yellow mangetout from The Real Seed Company (
www.realseeds.co.uk). Real Seeds have some other unusual varieties as well - I meant to try more but I haven't got the room!