I recently came across an interesting article on Irongeek.com (which itself is a pretty interesting security site that I'll probably add to my list of news feeds) entitled, "ALT+NUMPAD ASCII Key Combos: The α and Ω of Creating Obscure Passwords." The author suggests the idea of using non-standard (ie, not defined on standard keyboards) special characters as part of your password. It's common knowledge that adding special characters to your password greatly increases the difficulty of guessing or brute forcing the password. This extends the idea by adding normally hidden (and often unthought of) characters to the mix. So, while something like
abCD1234%^&* might be a good example of using special characters in a password (though obviously you'd want something more random than that sequence), consider this password:
äßÇÐ½²¶╔¥¢. I'd love to see the password cracker that can crack that one. :-)
Of course, as the author mentions there are downsides to this. Increased complexity notwithstanding, its strength is also its main weakness; these are non-standard characters, and as such not all applications and operating support them in the same manner (or at all). While this may work great as a Windows user password, for example, it may not be possible to use it as a Linux user password.
Regardless, it's still an interesting concept that deserves some attention. Check out the article for more details on the subject, as well as a tutorial and reference charts for entering special characters. The Wikipedia article on Windows Alt keycodes (also referenced in the article) is another good resource.