These are just for a start:
A very good and quite active (german) discussion board can be found at www.forum-64.de.
Another good (english) discussion board is www.lemon64.com.
For a very good C=64 tool-list check out The World of Fairlight
Another good place to start is the cosine page, check the links!
Other resources for games and demos are www.giga.or.at, c64.rulez.org, arnold.c64.org, www.c64.com and last not least utopia.hacktic.nl.
My website about the Commodore C65 prototype can be found at http://www.toxic-waste.de/c65/
That's it for now, more links to come!