Flash

Macromedia Flash is a popular technology for creating animated and interactive vector graphics presentations.
Your site should not depend on it. (some sites are a gigantic Flash page and that's not good.)
Flashy Flash ads are becoming increasingly more and more annoying.
Good uses for Flash:
1. Interactive or non-interactive Presentations, movies, Flicks.
2. As a replacement for Java.
3. To provide an alternative convenient interface to a site. (while still having a pure-HTML one)

Availability

The Flash client is distributed as a binary-only driver with a possibly problematic license.
The W3C Standard "SVG" (short for Scalable Vector Graphics) aims to be a non-proprietary alternative for Flash.
As of Mid-2004 it is under-implemented in most platforms.