If you are employing a Unicode solution to encoding and displaying Greek on your computer (as is strongly recommended), then all you need to be able to type Greek in your word processor, email client, etc., is a keyboard driver that recognises polytonic Greek input. Alternatively, you could use a palette, or visual keyboard, to access the characters you want.
The current versions of Windows (Windows XP Home, Windows XP Professional, Windows Vista) and Mac OS X (10.3+) come with optional keyboard layouts called "polytonic Greek." These are mostly sufficient for typing ancient Greek, but do not always represent certain punctuation characters (e.g., on one keyboard, the Greek semi-colon is missing, and must be accessed using a character map utility). They also follow the layouts of modern Greek keyboards, which can be confusing for English-speaking touch typists.
If you have purchased GreekKeys, LaserGreek, Antioch or another major font package, these will probably come with an included keyboard utility. If not, or if you prefer a different input method, the following options may be of use.
A professional tool for the creation of keyboard input layouts for Windows. Also includes links to several pre-written Greek keyboards:
A visual tool for creating new keyboard layouts for Macintosh OSX. Very easy to use--no doubt there are Greek keyboards out there that have been created with this tool?
A Greek keyboard input layout that uses pure Beta Code to type Unicode Greek. It is slightly unintuitive to have to type *a for a capital alpha, for example, but otherwise a good keyboard with excellent coverage of obscure symbols and beta escapes.
Andreas Schmidhauser has created new keyboard layouts for Greek (and Latin) for Windows. The keyboard layout and diacritic shortcuts are not as intuitive as the Classical Greek Unicode keyboard recommended above, but includes many more characters and sigla that may be of interest to classicists.
Some advice on selecting and installing Greek input tools for Linux are available from the Dimitri Marinakis at http://tlgu.carmen.gr/Hellenic_polytonic_HOWTO.html
There are several tools out there which will help to convert old Greek fonts to Unicode: see the Font and Unicode Converters section of the Tools page.