In all seriousness, buy Plants vs. Zombies
Everyone else seems to be listing mostly games, so I'll list useful apps - all of which are free, except for iSSH:
- Planets is a brilliant app and shows rising and setting times for your location for the planets and the moon, has a 3D view of the sky in visible, X-ray, hydrogen-α, infrared etc and has a 3D globe of each planet. And it's free!
- iSSH is great if you use SSH. It supports VNC too.
- MSN Messenger is... well, duh.
- Logitech TouchMouse lets you use the phone as a mouse to control a PC over the wireless network. Great for media centre PCs and presentations. You need to install a very lightweight server program on the machines you want to use it with.
- USB Disk lets you use the phone as a flash drive and lets you view certain kinds of files on the phone. It was free when I got it but they might be charging for it now.
- Remote Lite is a Remote Desktop client. You need the Professional edition of Windows (or Ultimate). You did get Windows 7 Pro, didn't you?
- Convert Units is a useful app for converting units, eg. area, volume, power, distance etc.
- Consume for checking usage for your ADSL, mobile data and calls, cable, home phone, wireless broadband etc.
- iPeng is great if you have a Logitech Squeezebox. It's a remote control and works similar to the Apple Remote afaik.
- eBay lets you keep an eye on your bids/auctions and is useful because it supports push notifications.
- xe.com currency is useful for checking the exchange rate.
- Pocket Weather AU Lite is free and is more accurate than the built-in weather app.
BTW if anyone could give me any hints to finishing the last level of PvZ that would be ossim... I've tried it twice and have been too lazy to try again for months