Your HTC Hero is a wireless receiver that is connected to a group of wireless networks. The 3G connectivity means it can surf the internet anywhere there is a signal. You know this--you probably do it all the time while out and about. But notice, the HTC Hero also has Wi-Fi. Anything with Wi-Fi must be able to send as well as receive information. So the question is, why can't the phone take that 3G internet signal and share it to a computer, portable gaming device, MP3 player, or anything with Wi-Fi or Bluetooth capabilities?
It can. This is called "tethering." It is when you use your phone to bring internet to another device. You may already be familiar with this and not know it; all the networks now sell separate devices that plug into any computer to provide an internet connection to that device--with the added price of a monthly plan. Tethering allows you to use your HTC Hero as that device. No need for another plan.
So how do you get in on this? There are two ways, wired and wireless. The wired one is easy, the wireless one is a bit harder. Here's a guide to each: