You can have a surprisng amount of fun with Skype voice changers. If you like to call your girlfriend pretending to be Barry White or want to call your boss disguised as Darth Vader, here are a few fun apps for Windows and Mac that show you how to change your voice in Skype:

Skype Voice Changer (Windows)

This simple and free voice changer for Skype. Simply install Skype Voice Changer on your PC, open up Skype and agree to the permission request. You'll then be presented with an effects panel that lets you alter your voice. There are a range of presets including Childish, Tremolo and Pitch Down and you can tweak each of them using the sliders.

MorphVOX Pro Voice Changer (Windows)

MorphVOX Pro Voice Changer has some interesting preset voices, including a demon, robot, woman, kid and even a dog! You can also apply background noises to your voice as you're talking, such as a shopping mall or a traffic jam (perfect for a call to your boss saying you'll be late for work). Another great thing about MorphVOX Pro is the 'Voice effects' section, where you can apply various filters to your dulcet tones, including nasal, growl, underwater and monotone. All of the presets can be customized using the 'Tweak Voice' control panel. MorphVOX Voice Changer costs $39.95, although you can try it free for 30 days. (More below the fold...)

Voice Candy (Mac)

Voice Candy labels itself as the 'Photo Booth of voice software' and although that's stretching reality a bit, it certainly provides some quick and easy voice effects for Mac users. There are eight choices of varying quality.  To apply them, you have to select one of the effects before recording, i.e. Normal, Chipmunk, Vader, Robot, Old Radio, Bad Science Fiction, Phone, and Sore Throat. Many of these effects are poorly implemented though such as the Bad Science Fiction effect which simply adds a few 'eerie' noises to the background. The Old Radio is a bit better however, as is the Chipmunk.

AV Voice Changer Software (Windows)

AV Voice Changer Software is designed to alter your voice - to make it funny or at least unrecognizable. AV Voice Changer Software can alter your voice live, as you speak into a microphone, and also alter recordings and audio streams. The demo is very limited in what you can do, but gives you a good idea of how it can morph your voice. The fastest way to change your voice is to plug in your microphone and click 'nickvoices', which gives you a range of presets. For example, if you're male, you can choose things like Baby, Prudent Girl, Schoolmistress or Old Woman. It has to be said that to our ears most of these sound fairly similar, and unrealistic. You can alter the pitch, tone and frequency of your voice manually in the full version of AV Voice Changer Software.

Garageband (Mac)

Garageband can be used to create funny and distorted voices in real-time as you chat on Skype courtesy of the Vocal Transformer. You will however need to download Soundflower first to channel your voice for Skype. You can also use it for online games such as Second Life. Full instructions about how to do so are here.