If you have another computer, iPhone, or iPod touch, you might be tempted to Google OS X Startup Key Combinations, which leads you to Apple's support document. With the information on this page you might be able to find a combination that allows you to restart your Mac and repair what ails it.
If, on the other hand, you have US$1.99 to throw at the problem, you can get the same information with Bayhoff Software's iStartup. As its name hints, it present a list of tasks--Force Mac OS X Startup, Reset NVRAM and PRAM, and Bypass Primary startup Volume, for example. Tap a task and the resulting screen presents the keystroke to execute at startup.
iStartup is helpful in that it's convenient to have these keyboard commands at your fingertips. How desirable it is depends on what you think your time is worth. This information is easily found on the Web and it wouldn't take a lot of time to key it into your iPhone or iPod touch's Notes application (and even less time to print it out and tape it to your Mac's monitor). But maybe your time is worth more than the measly $2 the author asks for the application. Only you can make that judgment.
iStartup is compatible with any iPhone or iPod touch running the iPhone 2.x software update.
[Senior editor Christopher Breen tackles troubleshooting questions in the Mac 911 blog.]