JiWire (www.jiwire.com): The easiest site to use -- some results even include a picture of the location to help you find it -- but also surprisingly comprehensive. A downloadable program lets you search this database offline.
NodeDB.com (www.nodedb.com): A little less polished than JiWire, but a good backstop to that catalogue. Especially helpful if you're going overseas.
Wi-FiHotSpotList.com (www.wi-fihotspotlist.com): Includes both free and paid access points; the latter heavily outnumber the former.
Wi-Fi Free Spot (www.wififreespot.com): A helpful resource if you want to look only for free access.
The Hotspot Haven (www.hotspothaven.com): Decent backup to the other sites.
WiFiMaps.com (www.wifimaps.com): Focuses on public-access hot spots, many unadvertised; best mapping options of any of these sites.
Capital Area Wireless Network (www.cawnet.org): Has a list of free hot spots; networking group that runs it can be helpful to users curious about the technology.
If you need to use WiFi more than a few times a month, it may be cheaper to sign up for a usage plan with a nationwide WiFi network that offers service in locations convenient to you. All offer monthly subscriptions, but many also sell prepaid service, in which you buy an allotment of connect time that can be used at your leisure.
T-Mobile HotSpot (www.t-mobile.com/hotspot):
$6/hour, $9.99/24 hours, $39.99/month unlimited. Available at Starbucks, Borders, Kinko's locations.
WiseZone (www.wisezone.net): $7.95/24 hours, $24.95/month unlimited; prepaid service available. Available in some independent coffee shops, Sheraton hotels, Baltimore-Washington International Airport.
Boingo (www.boingo.com): $7.95/24 hours, $39.95/month unlimited. Available in Cosi restaurants, Marriott, Wyndham, Sheraton, Hilton hotels, Los Angeles International and LaGuardia airports.
Wayport (www.wayport.com): $6.95 to $9.95/day, $49.95/month unlimited; prepaid service available. Available in Holiday Inn, Wyndham, Sheraton, Best Western hotels.
AT&T Wireless WiFi Service (www.attwireless.com/wifi): $9.99/24 hours, $49.99/month unlimited; prepaid service available. Available in Crowne Plaza, Hilton, Wyndham, Sheraton, Radisson hotels, plus Philadelphia International Airport.