jimbeetle - 2:16 pm on Jul 23, 2010 (gmt 0)
Okay, at least I found the marketing term for all this: Retargeting.
Code is inserted into a merchant or corporate page in order to serve a third-party cookie. DoubleClick uses a 1x1 iframe for this, fetchback dot com uses a 1x1 gif.
Navigate to a site serving DoubleClick or Fetchback ads, the cookie is read and the appropriate "retargeting" ads are served.