A question that comes up sometimes on e-mail lists devoted to matters related to ancient Greek is how to use a standard Latin character keyboard to write Greek characters and diacritical marks.
In Mac OS X there are a number of relatively easy ways to do this. Since version 10.4, a polytonic Greek keyboard has been included with the operating system. An excellent illustrated guide for installing and using it is available at Bryn Mawr's web site; they also provide similar instructions for Windows XP.
I personally prefer the free keyboard layout SophoKeys (Mac OS 10.2+), which uses the Beta Code transcription scheme of the Thesaurus Linguae Graecae. Beta Code is a widely-used standard and quite complete, including critical marks like the underdot (for a tentative reading of an unclear character) along with the conventional diacritics.
A widely-used package of fonts and keyboard layout for Mac OS X and Windows XP and Vista is GreekKeys. The price is $40 for non-members of the American Philological Association, which isn't unreasonable but obviously costs more than the free alternatives.
Adventurous Mac users can use SIL's free Ukelele application to create their own keyboard layout. Microsoft offers their own Keyboard Layout Creator for the same purpose, and Linux users can apparently use XKB.
Although it's been over a year since the site has been updated, Alan Wood's Unicode and multilingual programs and utilities is an excellent resource for more information on keyboards, fonts, and text editors/word processors.