AI_CIRCTAN makes it easier to place a circle tangent to three objects. This can be done with Circle, 3P, Tan, Tan, Tan; but this command speeds the process.

I have 2008, not sure it this will be available on the other releases.

Others to try:
AI_CONE
AI_DISH
AI_DOME

In fact, just type AI_ and start hitting TAB to see what is available that you may not know about.

Ciao.