I love open source software (OSS), even though, technically, I'm not a developer. I'm a consumer of oss, I guess you could say. At the heart of it, oss has had a profound impact on my development as an IT professional and hobbyist. Let me explain.
If it were not for operating systems such as Linux and FreeBSD and server software like the Apache web server and BIND DNS server. I would have had a real hard time gaining knowledge and experience building, tearing down, and rebuilding systems that I currently have. I spent many hours, as a hobbyist, playing with Linux, hosting a web, email, and ftp server from the living room of my first apartment when I was just 19. I gave people shell accounts and invited them to try to break my server. I learned a lot about TCP/IP networking, firewalls, security, and system administration during that time. The knowledge gained from those experiences helped me land my first systems administrator job at a local ISP, where I built and maintained a lot of those SAME services that I did as a hobbyist, and then some.
A lot of this comes from the fact that much of the oss is built on open standards that even proprietary vendors implement from time to time. The fact that you can learn the inner workings of standard protocols/services and how they are supposed to react in a given circumstance is an invaluable resource for an up and coming IT professional. It can help maintaining those proprietary systems that much easier.
It's safe to say that I wouldn't have near the knowledge or experience that I do today if it weren't for open source. Even today, I play around with oss. Currently, I'm experimenting with open source implementations of the nhrp protocol to better understand dynamic multipoint vpn services. I have a Linux desktop and utilize open source alternatives for desktop applications such as video editing and office productivity.
I wouldn't hesitate to implement a lot of mature open source solutions in a production environment. In fact, I find that in a lot of cases, open source software makes more sense and out performs a proprietary solution. Linux can be such a rock stable server operating system in the right environment. Add to it server software that is secure, stable, and reliable and you have a combination that can with stand the time of servers. Still, there is a time and place for oss in the business. Managers prefer commercial support for IT equipment and software. Some oss vendors reconize that and now offer those services to companies. Win, win! Thank you Open Source, both as a hobbyist and a professional.