Starting Points:

Palm OS Platform Development - The official Palm Platform Developers site.
http://www.palmos.com/dev/

Palm OS Programmers FAQ (formerly Wade's Pilot Programming FAQ) - A very comprehensive FAQ with many links providing a great starting point for learning how to create software for the Pilot
http://www.cyberport.com/~tangent/palm/faq/

Developer.com - Developer.com has links to many development resources for the Palm (and other platforms). Resources you can find include compilers, tools, articles, tutorials, etc.
http://www.developer.com

Darrin Massena's Pilot Software Development - Darrin Massena is heavily involved with Pilot development. He maintains a great site and even has programming newsgroups on his own server! Check it out.
http://www.massena.com/darrin/pilot/

C Language:

Andrew Howlett's Pilot Page - GNU Pilot SDK Page - Documentation & tutorials for the GNU Pilot development package mentioned below. Also has sample C programs for the Pilot with source code that can be compiled with GNU.
http://www.iosphere.net/~howlett/pilot/

GNU - Pilot C development package - A complete package including a C compiler, resource compiler, linker, and other tools for Pilot development. Available for free. Get it from Palm.
http://www.palmos.com/dev/tech/tools/gcc/

John Lehett's GNU home page - Info on the GNU Pilot C development package.
http://www.land-j.com/gccwin32.html

Pilot Resource Compiler - PilRC - Wes Cherry's Pilot Resource Compiler home page. PilRC is now maintained by Aaron Ardiri.
http://www.scumby.com/scumbysoft/pilot/pilrc/index.htm
/http://www.ardiri.com/index.cfm?redir=palm&cat=pilrc

comp.lang.c Frequently Asked Questions - Frequently asked questions about C programming from the comp.lang.c newsgroup.
http://www.eskimo.com/~scs/C-faq/top.html

Pilot DAT file format - Format of the PC based .DAT files generated by HotSync.
http://ourworld.compuserve.com/homepages/nseessle/