You may already have the tools you need. Modern DAWs like have a built-in "Pitch Correction" plugin. By setting the response time to its fastest setting and selecting the correct key, you can create a very convincing T-Pain-style effect without any third-party plugins at all. If you use a different DAW, search its user manual for "pitch correction" or "audio quantization"—you might be pleasantly surprised.
While the iZotope T-Pain Effect is a unique and powerful plugin, there are other alternatives available. Here are our top 5 picks: