The last generally release version of a smartphone OS released (almost entirely) by Nokia. Also, the first smartphone OS, by far. Also, for a long, long time and years after iOS the most popular smartphone OS by far.
UI Guidelines:
Nokia N9 UX Guidelines from Nokia. Seems to be oriented to developers, and none of the tools are re-usable templates, but lots of design examples. Check the other tabs as well.
Nokia Mobile Design Patterns A series of articles, approaching a pattern library. Note that some of these may be very familiar, as it is a wiki, so some older patterns from this book and other information posted to the wiki have been copied to here by the authors, and other users.
Nokia Asha UI Guidelines An intro to the Asha UI... but I am pretty sure that this ended up being just "Touch and Type" S40, so the Asha name is not important for UI design/development now.
Stencils & Templates:
Templates for smartphones meaning S60 3rd and 5th edition, Symbian ^3, Maemo and some newer S40 devices.
S60 Wireframing Stencils S60 (through 5th edition) elements of all sorts. (Illustrator, Fireworks)
Templates for mobile phones meaning mostly S40 featurephones, but also some older S60 devices.