It's the Palm Treo 750.
My very first smartphone, and probably the one I kept for the longest time ever.
It was running Windows Mobile actually (not PalmOne OS), and it was made by HTC. Still, it had Palm's great usability all over it, turning the old Windows Mobile 5 into a powerful and even comfortable smartphone!
The device felt perfect in the hand although it was a bit chubby. The keyboard worked well, and it was solid as a rock! (I dropped it many times and besides one keyboard button that didn't work so well – all the rest worked perfectly after a few years of using it).
Even today, from time to time, I enjoy holding it in my hand and remember the years when Windows Mobile and Palm were today's iOS and Android.