Official-ish modding resources

Official

api.minetest.net

Modding API reference of latest github master (your minetest might be older than this; check the "history" button in there for older versions.)
Modding API reference of latest stable

You can find your current version of the modding API reference locally, depending on the version, in:

Earlier:          mods/default/init.lua
0.4.dev-20120318: share/server/games/mesetint/mods/default/init.lua
0.4.dev-20120320: games/mesetint/mods/default/init.lua
Later:            doc/lua_api.txt
0.4.x:            doc/lua_api.txt

The default mods (once again the "history" button is useful)

World format specification also exists, if somebody needs it.

Community

Jeija's tutorial

And of course, you should take a look at everybody else's mods.

Let me know if there exists other quality tutorials or index pages.