Since it became available last April, the iPad has been a technological marvel.  Its touch-screen, ease of use and wide-range of apps make it a fun and functional device as well as a productive tool for business. There are over 15 million iPads in use today, according to Steve Jobs' March 2 keynote address. Despite its popularity, the truth is that none of us need an iPad. We just all want one. If you've considered getting one of the new models that came out recently, however, you may have been overwhelmed when you realized there are 18 different options. Here’s a run-down of what models are …