Add the Chrome extension "TeX for Gmail" (available from the Chrome Web Store). Compose an email in Gmail, type \lambda , select (highlight) that, click on the Tex button just to the right of the Send button at the bottom and choose "Guess Naive Tex". You can then copy and paste the λ character wherever you want. Having a Mac clipboard extension that has a history of what you have copied makes it more useful.
Of course it works for other Greek letters, e.g. \theta
Obscure but nice for those who already use Latex and or wanted to type it in Gmail on Chrome. Maybe it will be a gateway to Latex for others :)
Once you have this extension you can type nicely typeset math in documents providing you know or learn Latex.
Another obscure way, for emacs or Aquamacs users, type "C-x 8 RET 3bb". For those of you know Unicode really well :)
In any case different strokes for different folks.