The site that has all the info you'll ever need on this topic is probably Dave's Magical Macintosh Key Sequences (cache). He also links to this Apple KnowledgeBase (cache) Article.
Most important for me are (from the above documents):
Most important for me are (from the above documents):
| cmd-opt-p-r | Zap PRAM. Hold down until second chime. |
| cmd-opt-n-v | Clear NV RAM. Similar to reset-all in Open Firmware. |
| cmd-opt-o-f | Boot into open firmware |
| c | Boot from CD. If set to boot to X and no CD is present, may boot to 9. |
| d | Force the internal hard disk to be the startup device |
| shift | (OS X, 10.1.3 and later) Disables login items. Also disables non-essential kernel extensions (safe boot mode) |