As of Reason version 7 you can actually emulate the infamous 'wah-wah' paddle using the 'Line 6' amplifier.
In the video below and the audio sample to the right you can hear the dramatic effect this cool feature can give a track. The trick using this effect is to sway the effect from minimal to maximum in an un-linear fashion. Proper use and timing with this effect will create the typical 'wah-wah' sound.
If you have a prior version to Reason 7, you can still emulate this effect using the 'Scream 4' distortion module.