As a big fan of open-source software I was an early adopter of T-Mobile's Android phone the G1. Android is the open-source mobile phone software that is based on Linux and written by the development teams at Google. I've had it for about 10 months and I can tell you as someone who once had a mobile phone powered by Microsoft that it is a tremendous improvement.
A month or two ago other telecoms finally caught on, and Sprint and Verizon are now offering Android phones (as an aside - I used to use Sprint, until Android came out. Sprint at the time pointedly said it would NOT support Android - so I left).
The Verizon phone Droid, which is apparently much improved over my G1 (including Android 2.0, which apparently my phone won't run, and better hardware) has now sold almost 1 million units.
And Verizon is killing ATT in the network coverage wars. You'll remember that ATT sued Verizon over television ads comparing the two company's 3G coverage maps. Verizon initially said of the ATT suit that " the truth hurts." Now they've been proven right as ATT has dropped its suit and Verizon is swamping the airwaves touting its network advantage over ATT.