Many of them were created using Microsoft Gif Animator, a tool that is still used by the most nostalgic computer users.
In this application we aren't going to find advanced options, a wide range of image editing possibilities nor much usability. It simply has a gallery in which we input the images and select options to import color palettes and joining methods.
We will be able to edit the images dimensions and the time that each sequence lasts, as well as the color of its transparency. Likewise, we can configure the real dimensions of the animation, as well as the number of times that we want it to repeat.
There is nothing else. The strong point of Microsoft Gif Animator lies in its simplicity and how basic it is. If you want improved configuration possibilities you can resort to more modern and professional alternatives, but if you have reached this point, it is better to create an animation in Flash or Silverlight.
A tool that will delight nostalgic GIF animators.