Between work and personal, I own and use a lot of different phones.
My primary EDC phone at this moment is a tri-band Nokia 3650 (GSM 900/1800/1900), great because of the large screen and built-in 640x480 digital camera.
Also, when travelling I often carry a Siemens S46 because of its Multi-Band (GSM 900/1900 & TDMA [aka "Digital"] 800/1900) capability.
My Palm Tungsten W also has tri-band GSM phone capability, but I don't use that often because it works only with a headset.
Work also has provided me in with a like new Nokia 7210, a tri-band GSM beauty. Color-screen, speakerphone, wap browser, great RF, FM radio built in, tiny form factor, etc. While I love it, I think I'm covered in the GSM phone dept., so its for sale in the B/S/T.
Other phones I'll use on occasion, is a Ericsson R520m and/or Ericsson T39mc--both tri-band GSM golden oldies.
All my GSM phones are unlocked (i.e. can use any service provider's GSM SIM card). And my primary service provider is AT&T Wireless. When travelling I've used on occasion, Cingular, Swisscom, and Orange.