That would require an essay, rather than a post! Basically, every network of any size uses a mix of technologies, some open and some proprietary, from a variety of vendors. Even a "pure" Windows network will probably have Cisco devices and Linux-based appliances somewhere, even if the admins think of them as black boxes rather than specialized computers.
I'm guessing that you are interested in OSes... We have some Linux boxes, but integrating them into a mainly Windows network has been hard for three reasons: i) Active Directory integration is not perfect, and we have lots of user accounts ii) Running Windows and *NIX servers effectively means having to solve each admin problem twice because Windows is so different, and iii) most of the Windows administrators are not that interested in learning the *NIX way of doing things. To be fair, the administrator/server ratio is low enough that they can just brute-force any Windows problem without absolutely needing to switch to a more efficient OS, so the incentive isn't really there.
The second issue is by far the biggest headache. Linux servers can accommodate Windows clients one way or another, but Windows server apps usually assume Windows clients and often support other OSes in a token fashion, or not at all. To manage the two OSes you will probably need separate installation, update, and configuration management solutions, and possibly even separate backup solutions. I think that ultimately you need to pick one OS as the standard, and then work out a sensible compromise policy on how to handle the others without a big support investment.