Provide free access to the giffgaff.com domain and it's subdomains like community.giffgaff.com.
I would be very useful to top up my account using my card on giffgaff.com but from my phone and being freely able to do that without having to pay just to access the page where i'm going to give you (GiffGaff) more money/business. Same can be said if i want to contact an agent, contribute to the community while i'm out and about, learn of new goddybags from my mobile, check the pricing page, even just read the posts to maintenace issues that may explain why my service on that perticaluar day may not be working to its usual standard.
There are quite are few mobile networks and lots of MVNOs in the market now, one of the bigger players in the market 3 allow you to access the mobile and web version of three.co.uk and its subdomains and services with out charge from the users mobile.
I think it would be a very good feature for GiffGaff, a good feature for people to come to GiffGaff, plus a very good way to encourage further community particapation and also for use to spend more with GiffGaff.
I know i would spend more with GiffGaff if i didn't have to go to a shop to top up or find access to a PC that i trust with internet to top up, as i sure other users on GiffGaff are not always at home on their PCs or near any shops or any shops that allow to top up your mobile all the time.