Can anyone recommend a good distortion plug-in?

I am looking to add a distortion effect to my vocal track. I want something simple that will add distortion and not change the tonal properties of my track. If I am not mistaken, the Amp Sim plug-in is the only thing that comes close. The problem is that it kills tone (as it's meant for guitar), and it also does not have a fader to choose between the dry and wet signal. The latter is not necessary, but would be perfect. The most important part is finding something that is as transparent as possible, while still adding distortion.

it also does not have a fader to choose between the dry and wet signal. The latter is not necessary, but would be perfect.

You could add it to a bus and use a send to get just the right amount.

When it comes to vocals, parallel distortion is the way to go. As suggested above, a distortion bus works as well as a wet/dry control on the plugin itself. Better, actually, because you can route multiple vocal tracks to it and EQ/effect/automate them all together. I like to also add a delay or short reverb to the distortion chain.

The current king of distortion plugins is FabFilter's versatile Saturn, but it's expensive. If you're looking for a less-expensive solution, there are a lot of guitar amp sims that can do the job. I have used the bundled Guitar Rig, IKM's Amplitube, and some freebies such as Shred. 

But like you said, guitar effects tend to sound thin. For other applications such as bass, drums and vocals you need a less-specific distortion plugin that offers more options. Try the free Voxengo Boogex, which is a favorite of mine for bass but can do vocals if you tweak it right. Another freebie worth trying is CamelCrusher. A not-free but cheap one is Redopter from D16 Group, my go-to for parallel distortion on drums (although I've not yet tried it on vox; I should).

Then there are exciters and tape sims, which are great for vocals but may be more subtle than what you're after. If you have Ozone, it has a harmonic exciter that works pretty well on vocals, but it's closer to a tape sim than a distortion effect. The often-neglected FX Tape Sim is better than people realize. Toneboosters' ReelBus is cheap, but doesn't have a mix knob (or even an output slider). A decent freebie is TesslaPRO, which is similar to FX Tape Sim.

Awesome advice, looks like I have quite a few choices. Thanks for the screenshot bitflipper. To be honest, I have never messed around with different buses, but I think I understand what I need to do. Thanks guys!

Either way, you need, "this"

http://www.fabfilter.com/products/saturn.php

AmpliTube 3 FREE has some cool distortions and is stereo in/out for when you have stereo vocals. Usually on backgrounds where you pan Left/Right.

I use the cake tape sim when I need to add a little bit of low level grit to the vocal track.