The two Mac Pros were paid for by my dad's company, CERN. They were transferred to his ownership as part of his employment benefits. My dad's a physicist there and he uses the two Mac Pros for rendering work. The two Mac Pros are never in my home; they're in my dad's Geneva residence (I live in Queenstown, NZ)
The two iMacs were purchased to replace the elderly 2008 20" and 24" systems respectively. The 27" iMac acts as a home server and a gaming machine, as well as for running multiple VMs at the same time and 4K video editing. The 21.5" iMac is my regular desktop machine, for heavy photo editing and medium-heavy 1080p video editing.
The 15" rMBP (2.6/16/1TB/GT750M) was a free gift from my uncle (he's a software engineer at Apple). The base 13" rMBP (2.4/4/128) was won from a raffle, but I found it too underpowered for my needs and so my mom now uses it for web browsing, watching movies, iTunes and the occasional word processing. The other 13" rMBP (2.8 i7/16/512) was purchased as a lightweight but somewhat powerful machine for use outside of my home. I only carry the 15" when I need power outside of my home.
The early-2011 MBP was my main machine until it succumbed to Radeongate and got replaced by my two rMBPs.
The 2010 17" is for guest usage. It's permanently chained to the living room table with a Kensington lock.
The Mac Mini is mainly used as a HTPC as well as for web browsing and reading newspapers online in the living room. It's connected to a 42" LG Infinia Borderless in the living room.
The MacBook Air is my dad's carry-around machine.
So yeah, that's how all my Macs are being used