The first thing I would do with a new P900 is update the phone's software via the Sony Ericsson Update Service. It'll save you the hassle of backing-up the unit and then updating and restoring.
Other than that, there's a ton of great software out there. Opera browser, WorldMate, Handy Day 2004, etc. are all available via AllAboutSymbian's online store. One bit of software to avoid for the time being is Philips Camcoder, which is still not registering properly for paying customers, or giving full features.