This article at the New Scientist website notes that a number of major U.S. Internet service providers (ISPs) have been caught intercepting searches intended for Yahoo, Bing, and sometimes Google and redirecting them to to an advertising firm that would send the user not to a search results page but a sponsor's website. It's unclear whether they may also be inserting their own sponsored results into the returned search pages. Neither practice is legitimate, and both may be illegal. At least one lawsuit is in the works.
The known bad guys listed are
Wide Open West
In addition, Charter and Iowa Telecom were caught in the act as well but have now apparently stopped.
One way around this, at least with Google, is to use https rather than plain http at the start of the Google URL. The easiest way to do that is to bookmark Google and use the bookmark rather than your browser's search box, but unfortunately that's less convenient. The New Scientist article gives additional useful information.
Note that this behavior is distinct from another obnoxious practice of many ISPs, which is to intercept a mis-typed URL and take the user to a sponsored search page. This means that if you, for example, copy a URL out of a magazine and get one letter wrong, you pretty much have to reenter the whole URL because the ISP's redirection screws up the address box.
(Updated to reflect uncertainty about whether search results that are returned have been modified, and that this is distinct from the practice of intercepting bad URLs.)