If you have a Mac, all you need is GarageBand and Soundflower

GarageBand does some amazing tricks with MtF voice conversion; Soundflower is an old application (well, more like a framework) which sort of "pipes" you microphone through GarageBand, captures the output, and feeds it into whatever application you need.
I have a really, really deep voice, harsh and full of bass harmonics (as a smoker, this means it's even deeper!), and even though I've learned some tricks on how to project the voice to avoid being so full of harmonics (basically, not using your chest as resonation chamber, but just the head), it still is light-years away from sounding female.
Using GarageBand + Soundflower, besides casual voice chat now and then, I've successfully given two public lectures (remotely), as well as two interviews to TV stations. Nobody suspected that I was anything but a female

The results are really very, very good. The only disadvantage is, of course, that you need a powerful Mac to run all those applications, or there will be some digital glitches here and there. What I did to explain them away is simply to say that my computer is so old (true) and crawling to a stop (very true!) so that those glitches are unfortunately a consequence of my outdated hardware (not totally true, it's GarageBand in the background consuming all those CPU cycles). People shrugged it off most of the time and accepted the explanation.
On a PC I tried the AV Voice Changer software a couple of times, and I found it in general quite good.
In Second Life, the built-in voice morphing software also gives rather good results

but it's too expensive for my tastes (a monthly subscription)...