Internet access via built-in and addon airport cards are probably the easiest. If you don't have a built-in airport card, find out which airport cards your Mac supports first by using the Apple Spec database to identify your Mac and follow this article:
Once you have configured your airport software, use the menu for airport in the upper right corner of your menubar to connect to the airport network. Some may not work right away with your favorite web browser. Firefox has been able to get in on some open networks when other browsers can't. Once in, you can then get in on the open network with any browser.
If you find your Mac is not Airport Extreme compatible, but only compatible with the original airport card, see these resources for the original airport card:
A really good resource to find WiFi/Airport compatible hotspots on the road is http://www.jiwire.com/
EVDO is an increasingly popular cellphone service for wireless internet.
Getting EVDO on the road just became easier with Novatel and Spring EVDO USB modems:
EVDO is also avialable to Macs with both Express/34 slots (MacBook Pros) and PCMCIA slots (Powerbook G4 15", 17", and G3s):
For those who need multiple machines to use an EVDO connection, Kyocera has made this router:
Bluetooth also supports a variety of cell phones for internet access as these articles explain in the knowledgebase:
Use the above Apple Spec database to find out if your Mac has built-in bluetooth.