ohlordy I'm using two apps, both for different reasons:
Pregnancy Pro - I used the AF version of this for the past year and was great, so paid the £2 for the pregnancy one when I got my BFP. It shows your calendar with how many weeks you are clearly colour-coded. you can track weight, waist, symptoms, fetal movements, contractions, mood, notes and birth. Plus you can also record every type of appointment you have coming up in the calendar with alarm reminders. Each week it gives you a little blurb about what's happening this week,and you can track your weight/waist on a chart too. You can also store photos and name ideas too, but I don't use these features, I just use the vital stats and appointments features.
Bounty Pregnancy - Everytime you log in, it tells you exactly how many weeks+days you are, then gives you 6 options. 1) this week: whats happening in this week's pregnancy, plus loads of useful info about what symptoms you could get in this week and how to treat them. Info on food safety, exercise, antenatel visits and other stuff. 2) journal: you can keep a virtual diary here. 3) gallery: pics... 4) community: direct links to forums. 5) about: about bounty and the app. plus how to get in contact with the Tommys midwives. 6) settings: allows you to accurately put in your pregnancy details so the tracker is correct.
May seem silly to run two, but I find the info on the Bounty app useful reading, and the PP app great for storing stats etc.