I just re-read the ToS and since the link would be nether self promotional nor pornographic I think it's legal.
The tool I was thinking of is called Hitwise, which is very expensive. For small companies they have an offshoot product which does not track specific sites, but does track industries, that tool is called keywordintelligence and costs $90+ per month per industry you want to track. [I just checked in on the product and the basic package costs $90 and includes 3 industries]
This tool tracks the queries that a sample user base uses and then tracks the downstream site, so paid and unpaid listings are included without differentiation.
Since Keyword Intelligence does not break down individual sites the tools may not help you if your site does not fit into one of their many industries. The company I work for now, for example, does not and we need the full Hitwise package. There are many enough and diverse enough tracked intustries that most sites will find a good match, however.
[edited by: seth_wilde at 8:34 pm (utc) on Feb. 27, 2006]