This ad size is known as a button, along with the now defunct 125×125 ad unit.
A 120×60 ad unit is most commonly used on listing sites for freemium listings. They are also seen littered across the internet, placed in any spare gap on many websites, although this is not recommended.
120x60s generally do not cost much, as they do not perform very well. They are mostly ideal for co-branding excercises, where one website associates itself intrinsically with another via the use of a button ad instead of normal hyperlink.
This is how it looks:
120×60 Advice for Site Owners
While it is tempting to try and monetise every spare pixel on your website, 120x60s are not very profitable so should not be crammed into every spare space. If you really want one, having one on each page should be more than enough.
120x60s are useful for freemium listings, as they are something you can give away for free to encourage advertisers to spend with you (“buy a listing and get a free button ad worth xx”). This ad unit takes up little space, but can give the impression of added value to people who want a competitive edge on a listings site.
120x60s are also useful for co-branding campaigns. For example, if your site gets its coverage on a certain subject from another site, then using one of these buttons to advertise that is a good idea. It takes up little of your website space, and looks more official than a hyperlink.
120×60 Advice for Ad Buyers
Button ads have limited use when advertising yourself. Their best use is for co-branding exercises, but as these are ways of linking the brands of two sites, they should not usually be paid for. They are more for when it is in the best interests of both sites for people to associate them in their minds, so consider 120x60s for friendly collaborations more than commercial ventures.
We would not recommend spending any time or effort creating buttons or buying their ad space.