A quick search for ESTA application returned many results. According to this page on cbp.gov, the official site to apply for an ESTA is esta.cbp.dhs.gov.
Are the others scams, legitimate third party resellers or "added value" services?
- http://www.applyesta.com/ (defunct - domain is for sale circa 2020)
Can anyone confirm either way?
Updated 2020: The above have better "not affiliated with the U.S. Government" warnings now, but beware there are newer more sneaky sites starting all the time:
And fortunately more official guidance about this issue: https://nz.usembassy.gov/be-aware-of-esta-scams/