According to a , the hashtags achieved significant notoriety with Twitter users in 2007 during the San Diego fires, when users designated their tweets with "#sandiegofires."

The trend to use hashtags led to the community-driven site , where a semi-official index of Twitter's hashtags now resides. To access the site, Twitter users merely need to opt-in (for free) by following on Twitter.

If you're writing a tweet about a topic you think might have a unique hashtag assigned to it, you can visit to find it.

When you arrive at the site, choose "Tags" on the right-side menu. After the menu drops down, choose "directory."

Hashtags are listed alphabetically, but the directory won't be helpful unless you have a lot of time. It's jammed with obscure-looking hashtags (many are acronyms). As a result, you might have better luck searching the site for your hashtag of choice.