I'm not really able to post pictures right now, but a quick search for each of these names on images.google.com will bring up vast quantities of pictures.
Kenya tree - fast grower, waves nicely in the current, unusual "self-fragging" behavior spreads daughter colonies all over your tank. Cannot be isolated by keeping it on a separate rock.
Xenia - Comes in several varieties, from squat puff-ball shapes to long stalks. May or may not "pulse" in your tank. Everyone has a theory, but nobody can prove that any one factor makes them pulse. Spreads like wildfire in some tanks, sits quietly in the corner in some tanks, and dies immediately in others (another mystery).
Green star polyp - Despite the name, comes in several colors. Tends to spread to adjacent rocks, but can be contained by isolating its rock from any other rocks.
Anthelia - Looks like xenia, but grows from a base mat like green star polyps. Also tends to spread to adjacent rocks. Most common color is a basic salmon pink, but there may be a few other colors.
Zoanthids, palythoa and protopalythoa - Hundreds of colors to choose from. Named varieties with unusual colors often fetch ridiculous prices. Some spread aggressively, others slowly. Not actually soft corals at all, but often grouped with them.
Mushroom corals (corallimorphs) - Not as many color variations as zoanthids, but there are some bright ones (especially Ricordia). Most will spread, some quicker than others. Again, not soft corals as such, but often listed with them at online retailers.
Sinularia, Sarcophyton, etc. - Many, many types. Most are less colorful than the above, but with some exceptions (like neon green varieties). Generally don't spread as fast as the earlier corals in this list.
WetWebMedia.com is a great place to check out the various types of soft corals, with pictures of many varieties.
Edit: Unfortunately, many of the most colorful soft corals are not suitable for newcomers. Dendronephthea and Scleronephthea (lots of common names) are very pretty, but they're also difficult to keep alive.
