MorphVOX Junior is a free voice changer software app for Windows. Although it's designed for online games, you can use it for any purpose that requires voice changing and it works well with most VoIP and IM clients. MorphVOX Junior installs easily, and a relatively clean interface makes it easy to work with. There is a paid version of MorphVOX Junior that adds a lot more capability and better quality, but the free version will be fine for casual users.
Whether you want to change the timbre of your voice, make yourself sound like the opposite sex, or simply play with the various effects built into MorphVOX Junior, the system is easy to work with. While of higher quality than some other voice changers we've experimented with, this is still quite obviously a synthetic voice creation and is not likely to fool too many people (despite the advertising hype). A cool aspect of MorphVOX Junior is a library of sound effects you can easily trigger, adding a soundtrack to your calls.
MorphVOX Junior is more for entertainment than for real voice changing, and as an entertainment package it does fine. We used it several times, sometimes trying for subtle changes and sometimes going to extreme voice changes for laughs (you can make yourself sound like Darth Vader quite easily!). MorphVOX Junior is fun, if you can find the right uses.