Depending on what type of robot voice you're after - there are a number of ways to achieve said sound.
If you want a Dalek - sorry if you're not from Britian or Oz and don't know what a Dalek is...if so look it up, but a Dalek voice is created by using a ring modulator. Go to KVR - a website that has information about plugins. You should be able to get one for free. I use GR4.
If you want a more traditional type of robot voice then a vocoder would be the go - Wave's has one and you can pick it up for 30 bucks normally. I have this plug, and whilst it's not very intuitive to use...it is excellent still.
Then if you want something more esoteric, you could use a granular delay. Again, I use GR4 or Guitar Rig 4...the granular delay in the effects suite is excellent, but again tricky to use if you're just starting out. You can pick up GR5 - I think for about 90 bucks...but well worth it because it's the swiss army knife of effects.
Continuing down the granular track, you could also use a granular synth. I have Steinberg's Padshop...in reality Padshop is in fact a quantum representation of all time based effects - meaning in theory it can replicate every time based effect and more. Think delays and modulation. I like to think of granular synthesis as a bit like Willie Wonka's Glass Elevator. This is because a granular synth at its core is a sampler - but unlike a sampler, which can only go back and forwards - a granular synth can back, forward, left and right and any which way in between. Of course unless you're British you might not know Willie Wonka - his chocolate factory and of course the more obscure follow up book by Roald Dahl.
Finally, if you're reluctant in experimenting with above techniques...as someone has already said you could try a filter. An auto-wha maybe, fed into a distortion - maybe chuck on a leslie type speaker and some delay might do the trick. Again KVR will have a free leslie or invest in the 90 bucks for Guitar Rig 5 because it too has a leslie.
Hope that helps.