What's your favourite app for your smartphone?

Apps are IN! Everyone is building apps! Everyone is using apps! But how do you find the best apps for exactly your purpose?

As the market for apps are growing so rapidly, I find it more and more difficult to find good apps for my mobile phone. I'm trying to limit the amount of apps I'm using, and try to find just one app for each service (twitter, facebook, etc.) that I'm using.

Actually I have found that looking at other people's favourite apps, helps me find the right app for my specific purpose. Which is why I wanted to post this on my blog, to get your input on your favourite apps for iPhone, Android, Symbian, etc... It might also be helpful for others :)

Leave your favourite apps in the comments...