I think bacterium and viral life are perfect examples. I mean, life as we know it is based on the 7 processes we consider all living organisms to possess (namely: respiration, reproduction, sensitivity, growth, movement, excretion and taking in nutrition).
The best example I ever heard to challenge our views was something like...
"Bacteria do these processes but so rapidly that if they could perceive human beings they would see us as inanimate objects. They wouldn't be able to see us going through our processes because we would be doing it so slowly compared to them. What if a rock, or anything for that matter that we see as being inorganic, was simply a living organism that was just proceeding through it's processes as a much slower rate than we could perceive, millions or billions of years even."
And further more, we defined those perimeters. Does an organism need to be able to reproduce itself for it to be considered alive? What if they were reproduced by others? People are considered "alive" even when machines are breathing for them and pumping their blood. Sensation? That merely means being able to take input from the world around you and turn it into an output (aka its too hot here so lets go over there etc). Computers take in input and sensation all the time, in the form of basic temperature gauges or even keyboards. The 5 senses can all be utilised by machines e.g. webcams, microphones, touchscreen and pressure pads etc etc **http://singularityhu...e-in-5-years/**
To me, these perimeters can be met individually in many forms, we're just to close minded to think that anything that dosnt resemble our internal structure could match our magical gift of life or way more than that, our sentience, this idea that we know who we are and know that we're alive.
One day machines will question us and we'll be fucked.