Features:

• Requires an external viewer
• No sound (MPEG-2 allows sound, but there are not many players out there)
• Offers balance between size/resolution
• Players are available for most platforms
• Many people have players installed

Players:

• Sparkle (download for Macintosh)
• MpegPlay (Windows) (requires Win32s, and the link seems broken, so you'll have to check the search engines for this one)
• mpeg_play (Unix)
• Math2HTML (Unix-based Mathematica to HTML converter)
• img2mpeg (link goes to man-page of a tool written for use at the Geometry Center)

Encoders:

• mpeg_encode (Unix)
• Your suggestions for Mac or PC encoding tools can be inserted here...

Resources:

• MPEG Pointers and Resources
• MPEG Research at Berkeley