If you have been with O2 (Pay As You Go or Pay Monthly) for 6+ months then you can choose one of the following which lasts for 3 months, you then get to choose another one when it runs out.
1) Free Texts 7:00 pm - 8:00 pm Daily
2) 10 Free Any Network Anytime Minutes per Month
3) 20 Free UK Texts per Month
Simply call 2211 (Free) and chose your desired option.
I know this has been posted before (on the old site) but it is not being advertised now and the offer is still running. There are many people are still not taking O2 up on this free offer.