One mod installer there is, and the most famous one by the way, is MCpatcher. But it only allows you to install things like TMI and modloader, but it also makes your minecraft compatible with HD textures. Another famous one is nitrous, and this one also works to put GLSL shaders in your minecraft (super HD graphics card required, though). To run it, simply install it in your computer, and open it. minecraft server list It will tell you to browse for your minecraft jar file (WARNING: this must be a jar file, and not any other kind of file). When you locate your jar, (in your bin folder where it always is...) you have to open it with the installer, (some installers, like the zombe mod installer, locate the jar on their own, so try to look for one that does that for the easiest installing) and then select the mods you would like to install. Some installers require that you select mods that you have already downloaded to your computer, and others bring their own mods (again, like the zombe mod one). When they are done installing, sit back and enjoy playing minecraft with your brand new mods!

Note that some big mods can't be installed through installers, and you have to install them by hand, there are many YouTube tutorials on how to do this, though, so you'll be fine.