If you've ever had a friend send you a link that you just weren't sure about, webapp Variably Safe For Work (semi-)safely previews the page before you actually visit it.
To use the web site, all you need to do is paste your link into the textbox and click the Preview button, and you'll be shown a black screen with a slider bar. Slowly drag the slider to the right, and the web site will fade into view enough for you to be able to tell whether what you are viewing is safe for work or not.
If you really wanted to, you could create your own keyword bookmark that attaches the questionable URL to the end of the Variably Safe For Work URL instead of heading to the site every time. Either way, it's a very useful tool for those of us that constantly get inappropriate links from friends while we're at work.