Another vote for Ubuntu. As a desktop system, it is the easiest way to transition from Windows and OSX. If you're an OSX user, it'll be slightly easier to get into Linux mainly because of the familiarity in Terminal modes and UNIX/bsd based commands. Ubuntu also offers the Synaptic Package Manager which provides an easy way to install applications through deb packages without having to go through source code.
As mentioned before, you'll have to compile some applications from source, but those are dime a dozen these days. Linux has come a long way since it became popular in the early 90's so don't be afraid.
Keep in mind that you'll also be able to run some of your favorite Windows applications through Wine.
I've been running Ubuntu on one of my laptops for quite a while now and it's the most frequently used system in the house. Even the wife got used to it fairly quickly.