This is one of my favorite programs of all time. PDA Net allows you to use your Treo as a modem to connect to the Internet. You actually get a very good connection. The Treo 700 is significantly faster than the 650 in terms of internet connection speed. If you try this with a 650 and then upgrade to a 700, you will see a huge difference.
With this program, you basically get an internet connection anywhere you have cell service on your Treo. This is perfect if your phone has an unlimited data plan.
I hear there is a way to do this with a Mac too, using a Bluetooth connection, but I haven't seen it firsthand yet.