I've ordered a book by Riso about enneagrams (Wisdom of the Enneagram), but I haven't gotten it yet, so I'm supremely unqualified to comment about the subject.
That's never stopped me before, though...
so...
From what I've gathered from poking around on the Internet, the enneagram is a diagram which consists of a circle with nine equidistant points marked around it. Each point stands for a personality type. It sounds simple enough, but it gets a lot more complicated. As Macavity mentions, there are "wings," which are subordinate personality types. There are also a number of evolutionary levels within each personality type, depending on how positively or negatively one is expressing those energies. The whole system can be (and is) used to describe other things besides personality types; for example, some use it as a framework to describe spiritual insights.
It strikes me that there are several similarities here to astrology. The first and most obvious is that it provides a visual diagramming or mapping of the personality.
Secondly, as with astrology, the structure of the pattern provides restrictions on the possible permutations. For example, because of the physical layout of the solar system, one can't have the Sun and Mercury in opposition. Likewise, in the enneagram, for the "wings," it's specified that your secondary personality patterns can only be a personality type symbolized by an adjacent point on the circle to your main type. In other words, if you're a 3 as your main type, your secondary type can be a 4 or a 2, but it can't be a 7. (This feature, it seems to me, takes it out of the realm of objectively quantifiable psychology.)
Thirdly, as with astrology, the enneagram can be used in a deeper way to describe mystical or spiritual patterns of thought.
A fourth similarity would be that, as with astrology, your enneagram type is said to be permanent; you don't change from one type to another over the course of your life.
The major difference that I can see is that the "diagrams" of the various kinds of astrological charts (natal, horary, progressed, etc.) are imposed on us from the outside and are unchangeable (although the ways we interpret them and the ways in which we choose to express them certainly aren't), while with the enneagram, it's ultimately up to the individual to decide what type they are and how they fit into the diagram.
This leads me to wonder... astrology being such an excellent method of mapping the personality, would it ever be useful for someone to try mapping their own personality by taking astrological factors and consciously deciding what goes where in the chart? In other words, I have a cooperative personality so I'm going to put my Ascendant in Libra, or I strongly desire personal possesions so I'll put my Mars in the 2nd House, etc.... might be an interesting excercise...
-- Lee
edited to add fourth similarity