Within the tool:
"Help -> Default Keymap Reference"
(as suggested by krosenvold)
On the Internet: "IntelliJ IDEA Developers Documentation"
Windows, Linux and Mac keyboard shortcut reference cards for versions 8, 7, 6, 5, and 4.5.
(I knew I had seen and used these reference "cards" before! ;-)
One of my favorites (not shown on the JetBrains cards) is "Ctrl+Shift+{Up or Down Arrow}" to move lines/methods up and down in the source code.
See also "Intellij Idea Keyboard Shortcuts" page on Ward's Wiki.
