The iPhone's Calendar app is handy, but it frankly leaves a bit to be desired. If you'd like a more powerful, iCal-like interface for your iPhone, Week Cal is a great calendar replacement app.
One of the default Calendar app's biggest annoyances is its lack of a week view. Week Cal brings a iCal-like week view to your calendars, which you can easily organise, copy and paste, and drag and drop. Its list and month view are better than the default app's too, colour coding events on the month view (so you have a better idea of what's coming up) and even letting you pick the colours for each calendar. If you have any specific events you use often, you can create event templates for super quick adding, among many more cool features.
Probably the most convenient part of the app, though, is that you don't have to re-add any of your calendars. It grabs whatever accounts your iPhone is already using for Mail, Contacts and Calendars and imports them right into Week Cal — meaning you don't have to type in your username and password again. Hit the link to read more, or head straight to te iTunes Store to check it out.
Week Calendar is a $2.49 download for iPhone and iPod touch.