Okay, so these aren't all necessary, but they're very useful to have around! Many of these programs are needed to view or create multimedia. All the programs listed on this page are free downloads.
RealPlayer Basic This program is one of the most useful around on the Net. If you don't have it already, get it! :) Many of the videos and soundfiles you'll find on the Net need it. The videos and slideshows on this site as of writing all require RealPlayer 8.0 or above.
RealSlideshow Basic If you want to make slideshows, this is the simplest way to go about it. Creates relatively high-quality slideshows in a minimum of space.
RealProducer Basic So you've got videos, but the .avi files is huge. Encoding in RealMedia is the best way I've found, or at least the most Internet-friendly. The file sizes are small, but the trade off is that you get a bit lower video quality. Still, very, very useful for Internet video!