Lots of apps will do useful things, depending on what your interests are. But for the gee-whiz, 21 century, impress your friends app, I would recommend one of the "smart" apps which use rules/conditions to modify your phone's behavior. For example, wi-fi ON, ring silent, when at OFFICE. Or, ring at max volume when at your kid's soccer field.
I still use Setting Profiles, just because I got it when it first came out and know it.
Most people seem to like Tasker now. If I were starting out, I'd probably check that one out first.
The only one to avoid is Locale. It was the first in this genre and was great for a while. Then it went to a (very expensive) paid app and, at the same time, completely changed the code. Numerous complaints followed. If they had just kept the same code, they might have gotten away with charging so much, but the high fee and lousy performance killed the app. That is exactly when Setting Profiles first launched which also caused the demise of Locale.
Anyway, a smartphone needs a smart app.