I've added a link to a fine new resource on my Greek Fonts page:
By Randy Hoyt: a very useful tool, describing itself as "a web-based software tool that converts text from a standard keyboard into beautiful, polytonic Greek characters as you type. Using an easy-to-learn and standardized system called beta code, TypeGreek converts your keystrokes into Unicode-compliant Greek in real-time."
If you are not yet typing in unicode, this may be just what you are looking for. It behaves in the same way as the Unicode Classical Greek Inputter by James Naughton, which has often been mentioned on this blog (see all my entries on Unicode).
One minor comment: I find that the iota subscript only works if I am using a standard US keyboard, so to get it to work on my current computers, I have to switch to a US keyboard from my default UK keyboard. My guess is that that is a peculiarity of using a US PC with a UK keyboard, so this issue may be unique to the tiny set of people like me. (And if you want to know why I use a UK keyboard, it's because the @ is in a different place; likewise inverted commas and the £ symbol; years of touch typing on a UK keyboard makes the switch to US difficult).