You're not the first person who has asked me this
13" late-2013 rMBP, 2.4/4/128
This machine was won in a raffle, and is used for only word processing, web browsing, watching movies and light Photoshop.
13" late-2013 rMBP, 2.8 i7/16/512
This machine was purchased because the one from the raffle was underpowered for my needs. I use it for running multiple VMs, medium Photoshop work and some video editing.
15" late-2013 rMBP, 2.6/16/1TB/750M
This was a birthday gift. I use it for on-site 4K video editing (I'm a freelance photographer and videographer), as well as running multiple VMs and heavy image editing.
15" early-2011 cMBP, 2.3/16/512GB 840 Pro/1GB 6750M/antiglare display
This was purchased in 2011 as a desktop replacement, because my old Windows box had given up the ghost. I use this for image editing outdoors (because of the antiglare screen), as well as video editing and running multiple VMs. I like this model better than my 15" rMBP because of the antiglare display. The retina display attracts glare like a solar panel outdoors. It succumbed to Radeongate in March 2014, but a reball fixed it.
The 2010 17" was passed down from my dad, and is now used as a living room system for guest usage.
21.5" late-2013 iMac, 3.1 i7/16/256/750M
This was purchased for my desk, and handles heavy 1080p video editing and heavy Photoshop, running 2 VMs (4GB assigned to each) and some gaming.
27" late-2013 iMac, 3.5 i7/32/512GB/780M
This was purchased as a home server, mounted on a VESA mount on the wall. I use this for heavy rendering, 4K video editing and heavy gaming.
Late-2012 Mac Mini, 2.6 i7/16/1TB FD
This was originally my bedside PC, used for software development and image editing (yes, I work from my bed on lazy days), but it's now a HTPC connected to the TV in the living room. I still use it for the same purposes.
2 cylindrical nMPs, both with 12 cores, 64GB RAM, 12GB D700 and 1TB SSD
These aren't mine, but I use them when they're not being used by my dad. My dad's a physicist, so he uses it for heavy rendering and modelling.
13" mid-2013 MBA, 1.7 i7/8/512
This machine is used by my mom, but I don't exactly know what her usage on it is.