You can use any number of Voice changing applications which do work but the problem on the software side is that depending on the CPU load you have at the moment of transmission that application will (especially if it has lower priority and it should) introduce static and other noises. The good news with software is that just about anything is possible if you are willing to put up with everyone saying 'what?' from time to time. You can do a search for 'Voice Changer' or 'Voice Morph' or 'software vocoder' and most of them have free trials (not all FX are available in the trial versions).
On the hardware side they are called Virtualizers... Vocoders... Voice Tone Synthesizers and so on. I have seen one made for DJs out of New Jersey (GLI Pro)that sells for a little more than $50 but it is very limited and will not do radio static type FX. TC-Helicon produces a lot of different synthesizer pedals in the $150-700 range and their 'Voicetone Synth' (
http://www.tc-helicon.com/voicetonesynth.asp ) had me interested for awhile but ultimately I went with the Behringer Virtualizer Pro. TC-Helicons top of the line products (racks) actually do a better job but they also absolutely will cost you (the Voice Pro includes a ready made 'Electret Microphone aircraft cockpit' effect but its also $3000).
You can make your own effect with a lesser product but with hardware you are probably looking at a minimum of $300 (including mic mixer w/ preamp/phantom power virtualizer and cables). AND if you want to do midi FX with actors (actually emulate Tom Cruise or Ted Nugent or whoever) it will cost even more but it absolutely works with AH.

EDIT: I should also point out that changing FX with software means you have to minimize the game but with hardware... button pressing makes it a lot easier.
You can visit Partners In Rhyme and click on the 'Airplanes and Helicopters' section and you will find custom backgrounds for the FW190 Me-109 Mustang Spitfire or what have you. Im still learning how to mix that with the Virtualizer but I have already heard it used 'live' with Skype so I know I will eventually have a custom FX put together.