Well I've got a mixed estate of 8 Pis
1) is a motion sensitive camera overlooking the back of the house, a temp. sensor reporting to pi2 and an in-house web-server
2) is a file server, iplayer server, runs a daemon that controls 4 remote wireless mains sockets and records temp. measurements from around the house.
3) Has daemon controlling external house-lights via a Power MOSFET, has a temp. sensor reporting to pi2.
4) Motion sensitive camera and PIR sensor.
5) Motion sensitive camera, has a temp. sensor reporting to pi2
6) Controls via Pi2 a heater in otherwise unheated utility room, has a temp. sensor reporting to pi2.
7) has 2 temp. sensors reporting to pi2.
8) testbed - at mo' has a Schmitt trigger light sensor tracking dawn-dusk
Very simple jobs which could be combined into a smaller number but would mean swathes of wiring around the house.
Combining the capabilities can be usefull. Just tracked over the last few weeks an area of loft that had an intruder that sounded
too loud to be a mouse. So PIR detector switched on light and motion cameras spotted a mouse and a few days later noted its demise.