Data URLs are a facinating way to reduce the number of small data requests — to transmit small images along with the main HTML document, for example. The following applications support data: URLs:
- Mozilla
- QuickTime
- w3.el for Emacs
Example (please view source): 
Update: Phil Ringnalda points out that since Microsoft IE doesn’t support these they aren’t as useful as they could be. Which is true. And I should clarify that there is a size limit to the length of the URL. SGML limitations mean that the maximum length is about 1K, so this will only work with small images.