Here is a cool plugin that is free and do some great inter-aural delay tricks to widen stuff up. These delay tricks are also known as the haas effect as someone has mentioned already.
http://www.voxengo.com/product/stereotouch/
Another trick I like to use is some sort of auto panner set to go side to side by a modest amount.
One thing you need to be careful with though is phase cancellation with using delay tricks. Unless you don’t care about your mixes ever being heard in mono then it helps to check it in mono. There are some club systems that run in mono.
Also if the content already has some stereo information then you can use a mid side EQ and boost the gain of the sides(the stereo information) and that will give the perception of more width as well.
edit ::ok sorry I said a lot of what people said already. That free plugin is good though I use it a lot.