Sayz Me is a freeware, Windows-compatible text-to-speech utility. Though the tedious installation process does involve three separate EXE files, once you have it up and running on your system, Sayz Me does a great job taking text and reading it back with unusual, robotic voices. Sayz Me offers users the ability to choose among a variety of different voice timbres, then adjust parameters for both pitch and playback speed.
Audacity is a freeware, open-source audio editor and processor compatible with OS X, Windows and Linux. Audacity comes with a variety of built-in filters and effects that you can use to take a recording of a voice performance and make give it a digital or robotic quality, including echo, pitch shift and wah. In addition, Audacity loads loads 32-bit VST effect plugins, which expands it basic capabilities. Consider applying a tremolo effect, a band-pass filter or a high-frequency, tempo-synced delay with a short decay and no feedback.
Togu Audio Line's TAL-Vocoder, a VST emulation of a 1980s-era multi-band vocoder -- the type popularized by German electronic pop pioneers Kraftwerk. TAL-Vocoder can take a pre-recorded voice and give it a digital character. Togu Audio Line specifically designed this plugin with vocals in mind, and included a few parameters meant to keep words intelligible even after heavy processing. TAL-Vocoder works with Windows-based VST hosts and on Intel versions of OS X.
Image Line's FL Studio software sequencer comes with a collection of proprietary plugins, including Fruity Vocoder. Like TAL-Vocoder, Fruity Vocoder aims to emulate a vintage hardware vocoder. When used to process voices, Fruity Vocoder can create a variety of unusual effects. FL Studio Producer edition also comes with a second vocoder plugin, Vocodex. Though designed for Windows, Image Line has announced a beta test for an upcoming port for OS X (via Crossover). Some versions of FL Studio work on Linux via WINE.