Re: The Purpose of the Ring
Derek McG.

After reading Nina R's post, I went back and re-read the passages from The Silmarillion relating to the Rings of Power.  I found the following interesting quote: 

"... the Noldor desired ever to increase the skill and subtlety of their works ... and they desired both to stay in Middle Earth, which indeed they loved, and yet to enjoy the bliss of those that departed."

The power of the Elven Rings is in preservation - they protect the Elven lands from the ravages of time and preserve that which the Elves love.  It is due to the power of Galadriel's Ring that "on the land of Lorien no shadow lay".

It seems to me that the Elven Rings were forged in order to re-create, in Middle Earth, the paradise that the Noldor had experienced in Valinor (without actually leaving Middle Earth, which they had grown to love).  In essence, the Elves wanted to have their cake and eat it too.

As for the purpose behind forging the Rings of Dwarves and Men, I have no clear idea.   Little is said of them in either Lord of the Rings or The Silmarillion.  It is clear that the Rings give power according to the stature and nature of the bearer, but I do not know exactly how they were used by either Dwarves or Men.