I've been a long-time Windows user, and one of the many remaining traces of the old days lies deep within the accessories menu. I speak, of course, of the character map, something I rarely pull up except to find things like © and the occasional £. Apparently one of the hot trends on Twitter is adding some of these barely-used symbols to your Tweets, or it's at least enough of a trend for two guys over at The Next Web blog to create their own bookmarklet to avoid having to open up the character map, or learn the keyboard shortcuts that spawn them.
Since it's a bookmarklet, it will work on any browser, so you don't have to worry about any of the compatibility or slowdown issues usually associated with plug-ins. When you click it, it's simply a pop-up with 60 different Web-friendly symbols that should display no matter what browser or operating system you're on. You just copy and paste the one you want and you're done.
There are actually some benefits to using these symbols as opposed to text. Considering the short-form nature of the service, you can save a character or two with the proper symbol. However, the vast majority, including ☭, ☃ and ☄ will be useless for most people.