Apple's Garageband is one of the easiest software programs available for mixing and adding effects to music. Garageband comes installed in most new Macs, and offers an array of digital effects.
Considered the recording industry standard, Digidesign's ProTools is one of the best software programs available for effects and mixing. The software is compatible for Macs and PCs, and comes with an array of pre-loaded effects plug-ins and a multi-functional mixing suite. ProTools mixes can be automated.
Apple's Logic Studio is a recording software platform that enables mixing and effects add-ons in a relatively user-friendly presentation. It includes modeled and sampler instruments as well as an array of loops.
Steinberg's WaveLab is a professional music editing and mastering program that includes a variety of effects and mixing capabilities.
Steinberg's Nuendo platform runs on Mac and PC systems, and comes loaded with automated mixing and an effects plug-in suite.