The two major plug-in effect types are VST and DX.

Steinberg's VST (Virtual Studio Technology) is an interface standard for connecting audio effect plugins to audio editors and hard-disk recording systems. VST and similar technologies such as DX (DirectX) effects allow the replacement of traditional recording studio hardware with software counterparts. Thousands of plugins exist, making VST the most widespread audio plug-in architecture.

DirectX effect plugins were developed by Microsoft as part of DirectShow; architecturally, DirectX plugins are DirectShow filters.

The benefit of 3rd party plug-in support (apart from the obvious) is that it has allowed the developers of host applications (such as audio editing and mixing software) to focus on the functionality of their products while specialists in digital signal processing concentrate solely on effects. This division of labour has resulted in a vast array of extremely powerful effects plugins that allow the home PC based studio to compete with its professional counterpart. There are plugins for practically every effect and every mixing and mastering process.

Note: VST effects are different from VST instruments (VSTi) which are midi-based instruments for software synths.

The best sites for free VST and DirectX effects are:

• KVR Audio: the most comprehensive source for audio effects plugins; if it exists, it's almost certainly featured in their plugins database.
• Audio Mastermind's free VST plugin section.
• Don't Crack's free VST plugin section.
• Shareware Music Machine's "latest software" section includes new VST submissions as well as other software which may be of interest.