Twitter Keys is a little bookmarket you can install so you can insert basic Western emoticons into your Twitter messages. As they point out, “☁ out, better bring ☂” takes up many fewer characters than “Cloudy out, better bring umbrella.”
It basically is an online version of the Windows Character Map and Macintosh Character Palette, but marketed for Twitter (very clever). BUT…it does work in any tool which works with UTF-8 text, including this blog, Facebook, Google services, etc. It technically works with offline tools like Microsoft Word, and it probably does have the most important emoticons handy. I may end up ♥ing it or at least ❥ing (liking) it.
FYI – If Twitter Keys is missing a desperately important character (not a lot of foreign language support IMHO), you can use the native accent tools on your Mac or PC on Twitter and many other tools.