ModLoader was a mod manager and organizer created by Minecraft Forums user Risugami.
Risugami's ModLoader was created in early 2010, and was supported until Minecraft version 1.6.2. Risugami's ModLoader was considered the best of its kind at the time. Its goal was to help eliminate conflicts between mods, which was incredibly common when multiple mods were installed at once. For example, before 1.9, Minecraft ran on a system whereby every item and block in the game had a unique ID number. When modifying Minecraft, if two mods added a block or item with the same ID number, the game would fail to run. ModLoader helped eliminate this. Because of this function, and many more, ModLoader became the premier piece of software of its kind, resulting in almost all mods requiring ModLoader as a dependency. Many veteran programmers used ModLoader to build their mods off of. The earliest versions of IndustrialCraft² and BuildCraft, two of the first big technology mods for Minecraft, required ModLoader.
As time went on, Risugami was taking longer and longer to update his ModLoader. Many believed he had fallen out of love with the game, but felt a continued obligation to update his ModLoader for the community. During the release of Minecraft version 1.4, Risugami took longer to update his ModLoader than some members of the Minecraft Forge team would have liked. This led to the Minecraft Forge team feeling like it was best to create a mod loader for themselves, which ultimately led to the creation of Forge ModLoader.