You can get one from my site here: http://trafficclose.xlphp.net/autohits.zip That is a common autohit exchange script you will see used for several exchanges. You might also check out this one: http://www.turbotraffictrader.com/ I have never tried it, but it is free.
good question! I looked around forever for a good free one Finally I found it Here it is: http://linkex.dk/ See mine in action here: http://www.justinchelf.com/wp-trade/ p.s. you dont have to use it for wordpress my secret