First, a bit about my background.. I currently work as a Network Engineer, mainly with Cisco routers/switches and VPN devices. I am throughly familiar with Windows, both as a workstation and server, and have one more test to go to complete my MCSE. However, I have no real experience with Linux, and feel like this is both a hole in my personal knowledge and in my resume. 
I can do basic user stuff both from a command line and in Xwindows, but have no real command of it.
That said, I intend to rectify this. I want to start using Linux as my primary operating system on my personal PC, as I feel this is a great first step to really force myself to learn it. I need to know what distribution would be best for me to use. My concern is some of them seem to have alot of propriatary tools that are not found in other linux distros, I want to find one to learn on that will best allow me to be comfortable on any linux distro I run into in the future. Thanks much for your advice!
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Linux truly is a wonderful operating system! If I'm using a PC, I greatly prefer using it to Windows. It's much more stable, secure, etc. But you already knew that. =]
Personally, I would suggest you start out with OpenSuSE. It's easy to use, yet it is very powerful. Its installer is also pretty easy. I recommend that because a lot of businesses that run Linux on their client systems use SuSE (the only difference between that and OpenSuSE is that SuSE contains closed-source programs; therefore, you have to pay for that distro).
I hope this helps! Good luck!