ModLoader is a popular Mod installer for Minecraft. It makes it much easier for those without technical knowledge to install mods in Minecraft.
To initially install it, the process is a bit tricky but afterwards it makes life much easier. To install ModLoader, go to the Start Menu and select "Run". Type in “%appdata%” and you'll be presented with the .minecraft/bin folder. You must then open the minecraft.jar with WinRAR or 7Zip. Finally, drag and drop all downloaded files into minecraft.jar. The last step is to delete the META-INF folder. If you have any problem, there's a handy How to install ModLoader tutorial video to help you out.
ModLoader makes installing mods into Minecraft much easier.