I am curious to know whether PhD is the highest degree one can earn? Is there any other degree beyond that, if so what is it and what are the universities offering the same?
The degree you're looking for (at least in the sciences) is called "Doctor of Science" and you can read all about it on Wikipedia:
Edit: As aeismail noted: Sometimes this degree is considered equivalent to a PhD and sometimes it's considered beyond a PhD. This distinction typically varies by country (all this is listed on the Wikipedia page).
At the risk of answering a question that the OP may not actually be asking....
One of the biggest myths to deprogram grad students of is the idea that the Ph.D is "the highest degree" or is in fact in a well-defined ordering relationship with other degrees. It is true that a Ph.D will typically require other degrees as a prerequisite, and it is also true that (as far as I know) no degree program requires a Ph.D for admission. However, there's no useful sense in which the Ph.D is "highest" in anything. It is a certification that you can do research, and is almost always a mandatory step before getting a research position in academia.
But by that logic, an MBA, a J.D or an MD are also "highest" degrees.
Many countries have higher degrees than the PhD.
In the UK, there's
- Litt.D Doctor of Letters / Literature
- DSc Doctor of Science
- LL.d Doctor of Laws
- D.D. Doctor of Divinity (the highest)
Each of these typically requires the submission of a body of work - a research portfolio - together with a critique of the work. Or they may be awarded as honorary degrees; see the links above for the requirements for the degrees from the University of East Anglia (Litt.D, DSc, LL.d), and the University of Oxford (D.D.), accordingly.
I heard that in Russia they have an equivalent to the standard Ph.D and a sort of "second Ph.D" after that, that is way harder to get than the first. One of my Russian friends argued that it is harder and rarer than the European "habilitation", but I guess it is open to debate to see whether it is just a matter of opinion.
Here is the corresponding article in Wikipedia: http://en.wikipedia.org/wiki/Doktor_nauk
(On an unrelated topic, I think the name is quite cool.)
In Finland, someone with a doctor's degree and additional body of scientific work equivalent to another thesis can apply for the title of docent (dosentti). The title gives the right to mentor PhD students. Docenture is a degree in the sense that it does not imply employment at the university which bestows the title.
At least some professors consider it kind of a vestige and it might be phased out at some point.