One of the cool things about buying an unlocked phone - as opposed to a handset branded and programmed by your service provider - is that you get access to the handset's original operating system. Unbranded phones are almost always more customizable than their carrier-supported counterparts, and quite often allow for greater user access to features.
Unbranded phones also give a glimpse into a manufacturer's soul by way of their choice of operating system and user interface. If you're interested in the most robust, feature-packed phones out there, take a look at handsets running the following OSs:
UIQ 3 / Symbian 9.1
Sony Ericsson's p990, m600i and forthcoming w950i all run the beautiful, icon-driven UIQ 3 interface atop Symbian OS 9.1. The collapsible "Today" standby menu gives direct access to calendar and messaging information and customizable shortcuts, while the platform allows for multitasking and robust media performance. Though not as popular in the US as overseas, Symbian is a flexible platform with widespread support from users and developers alike.
Series 60 3rd Edition (Symbian)
Nokia's Series 60 interface offers a different face for Symbian smartphones. Featuring an active standby menu system similar to UIQ's "Today!" menu, Series 60 provides all of the Symbian goodies (shortcuts, icons, multitasking) with a clean, business-like look. S60 3rd edition is much zipper than previous versions, and it powers the amazing Web browser on Nokia's E70.
Windows Mobile 5
The most popular OS for smartphones in the United States, Windows Mobile 5 is the latest mobile phone OS from Microsoft. That means you get integrated syncing with Microsoft PCs and Exchange Servers, which is huge for corporate users and IT departments alike. That also means you get the Microsoft user experience, which works just fine but isn't as "cuddly" as UIQ or "sophisticated" as S60.
Something of a renegade in the computer world, Linux has made its way onto a few mobile devices, including Motorola's "Ming" A1200 smartphone. Linux is the OS of choice for DIY hacker types, and if the newly rumored FIC GTA001 phone (complete with Linux OS, touchscreen, and developer kit included) is actually released, the notion of Linux on a phone could gain some traction in the open-source community.
Once the king of the PDA, Palm is an interesting place when it comes to mobile phones. Palm makes the Treo, so Treos run Palm OS, right? Well, most of them do, anyway. The Palm 700 and 750 are now available with Windows Mobile as well as Palm OS. It remains to be seen if this is a way for Palm to bring some Treo love to WinMob users or a baby step towards the end of the Palm OS.