Some harmonizers can operate in real time, meaning that they are able to produce and play back preset harmonies simultaneously with the lead vocalist's performance. These are often used live, making it possible to improvise harmonic combinations on the fly, or used privately as practice aids in the absence of actual back-up singers. Other harmonizers use only prerecorded vocal tracks, which they can harmonize with greater precision than is usually possible in a real time context. Harmonizers like these are typically used in post-production, after all the recording for a song is complete.
A software-based harmonizer usually takes the form of a plugin for use in a D.A.W. ("digital audio workstation") environment such as Logic Pro, Pro Tools or Ableton Live, though some can also function as stand-alone applications. Prominent examples include Celemony's Melodyne, Mu Technology's Mu Voice, zplane's vielklang [sic] and Antares's Harmony EFX. Of these four, only Melodyne is not capable of real time harmonization because it was designed specifically for editing prerecorded material. Overall, these four harmonizers are comparable in sound quality to hardware-based harmonizers, while being considerably less expensive.
With the rise of software-based harmonizers, hardware-based harmonizers began to decline in use. This is partly because of cost, but also because hardware-based harmonizers are complex devices made specifically for trained recording engineers, meaning they can be difficult for amateurs to learn and use. However, they are still found in studios that do not use one of the major D.A.W. environments mentioned above. Eventide has produced hardware harmonizers since the 1970s, and as of 2011 still produces several models, such as the H8000FW and the H7600 (see Resources).
If you plan on using a harmonizer, whether it be software- or hardware-based, there are a few further matters to take into account. Some harmonizers are quite user-friendly, and come with easy to understand presets for inexperienced users. Antares's various harmonizer plugins fall into this category, as well as several of the lesser known harmonizer plugins, like Prodyon's Enchoir or MeldaProduction's MMultiBandHarmonizer. However, some harmonizers -- including hardware harmonizers and some software-based harmonizers like Melodyne and vielklang -- are designed with experienced users in mind. Make use of the KVR Audio Plugins database for descriptions and user reviews of all available harmonizer plugins (see Resources).