You don't need an iPhone to try out iPhone apps. Since the application platform is just a browser, you can see right now how some of the apps that people are building will look on your phone.
We've covered Digg and OneTrip already, but to see even more iPhone apps all together on one page, check out iPhoneApplicationList. For best results you should use Safari, although I've also had good luck previewing iPhone apps in Firefox.
Since the apps are just Web pages, no doubt there will be many, many iPhone app directories springing up soon.