At the moment, nearly all of these options are limited to various versions of the Netscape browser only. Right now, the best way to create simple animations that can be seen by any user of Netscape 2.0 for any platform without breaking other browsers too severely is probably GIF89 animation. This is an especially good solution because these multi-part GIF files simply display their first frame (and do not animate) on browsers that don't support animation. This works because the GIF89 standard has included the little-used multi-image feature for a long time and existing GIF viewers know how to at least ignore the later frames. This is an easy way to achieve backwards compatibility with older browsers.
One tool that can be used to create such GIF89 animations on the PC platform is GIFMake <URL:http://www.fastlane.net/~samiel>. Tools are also available for other platforms (submissions welcome!).
One note of warning: Netscape always delays one-tenth of a second between frames, regardless of the frame delay you set in your GIF89 file.