Lots of new ones to try here - thank you. The ones I really find useful are My Recipe Book (hardly use a real recipe book now, so easy to add recipes and create shopping lists), TouchNote for postcards, BBC News, Duolingo, ColorSplash and iPhoto for photo editing, UK Maps because you can download very detailed Ordnance Survey maps to overlay onto a normal road map for driving / walking, Poems by Heart - you can add your choices. I love Ski Tracks because I can monitor how far, how fast, how many lifts etc per day and then see them plotted on a satellite view, and Snow Forecast. ViaMichelin for travelling in France.
Migraine Diary is useful for me. So is British Red Cross First Aid. BT Wifi for getting online when I'm out.
Sorry there are so many but I use them all, all the time!