Learn English – the origin of “cool beans”

Random House: http://www.randomhouse.com/wotd/index.pperl?date=20010223

The OED gives a 1574 quote for spill it meaning 'to divulge, let out': "Although it be a shame to spill it, I will not leaue ['omit'] to say that which..his friends haue said vnto me."

The spilling of beans endures, within and outside the U.S. (...) Over the years, there have been countless variations of form and meaning, all with spill. (...) And although we have tracked spill, we still don't know beans.

A popular folk etymology for spill the beans claims that in ancient Greece, applicants for membership in secret societies were voted upon by having the existing members drop beans into an opaque pottery jar.

From thefreedictionary.com:

15 Marked by full control over elements or subordinates; firm: tight management; a tight orchestral performance.

In music it usually means all members of an ensemble are playing together with rhythmic and harmonic precision. I think it's not much of a stretch to work from that meaning to the sense of something being remarkable or "slick".