Macs already have screen sharing built in; just look for VNC apps for the phone.
What sets apart the good ones are the controls, since controlling a Mac screen with an itty bitty iPhone is a bit painful. I like Remoter VNC; it's a nice app and relatively inexpensive.
Logmein and TeamViewer are services; they can make it somewhat easier for people who don't know how to use the built-in sharing capabilities of their Macs, or how to share various services over the internet.
And if you don't need to control the whole screen there are a bunch of remote control apps; Apple makes one for example.