I use statcounter along with domainname.com/coupon# and I know which postcard or advertisement they used to come to the site. I also have several 1-800 #'s I use and can track which ad I put the number on.
I use a tool called AdLog24 (w w w . adlog24 . com). It's relatively new, and I like the easy handling.
Via a tracking URL generated by the program itself, it counts the clicks on a banner, advertising link, or whatever you have.
It's much easier to place the tracking URL before your actual URL than to put tracking code in your source text (as you have to with Google Analytics, for example).
Tracking up to three campaigns is free, four or more cost a bit, but it's affordable, IMHO.