I have never felt the need to try one of these, now I'm glad I haven't:

A Very Scary Company.  One thing I wish the EFF should do.  Why don't they blow the trumpet on comScore?  This company has tricked millions of people into "download accelerators" and other trojan horse software that tracks their traffic, credit card usage (it actually captures numbers), and more -- all in the name of so-called research.  Most of the download accelerator software providers they use are front companies. [John Robb's Radio Weblog]

I'd always wondered what these so-called "accelerators" did, and how they actually did it.