S60 (formerly Series 60)
Once known as Nokia Series 60, S60 is the user interface used on a number of Nokia (and LG Electronics, Lenovo, Panasonic, Samsung, Sendo, Siemens et al) smartphones.
It sits on top of the Symbian operating system, much like the UIQ user interface used in the various UIQ Phones. Sony Ericsson M600, P800, P900, P990, Motorola A920, A1000 (and others).
The S60 Getting Started and UIQ Getting Started pages on this wiki should help you get off the starting blocks with these devices.
Information pertinant to particular S60 phones can be found on the Specific Phones pages, whilst the Must Have Apps page should help you find the best applications for your phone. Series 60 Tips and Tricks covers some useful things to get the most from your S60 phone.
The Symbian Development pages cover or point to some of the most useful resources for development on the Symbian platforms.