The three ones that I use are ExpressPCB, Kicad, and Cadence/Allegro. Of all, ExpressPCB was the easiest to learn, but is limited to making boards thru them (yes, you can buy Gerbers after you made a board for another $65). I have just stated using Kicad, and it looks like a very nice package, and well worth the cost (free!). Cadence is extremely powerful, has a long learning curve, and costs thousands to license so I only use it at work :-)
Another one that's very popular is Eagle; I haven't used it myself.
Since starting to make my own PCBs years ago, I will never go back to wire wrap...
/mike