An 802.11b or 802.11a wireless network card -- PCMCIA for a laptop, PCI for a desktop. Around $50. That'll allow you to log on to most existing wireless networks (there's a new standard just now coming out, 802.11g I think, faster but more expensive.) If you want to set up a network yourself, you also need a base station/hub/router, which'll run $100, maybe less.
Yes and the speed is great contrary to what a lot of people say. I have a wireless network at home with a desktop and two laptops and I will tell you I love it. Buy linksys, they are the best. Get USB network adapters too. They rock!