Coloured glass, mostly vintage/mid century pieces that have a 'use', even though I don't use them 
I build a collection of a couple of colours at a time until it feels there's enough of those colours. I've got green, turquoise, purple/pink, orange, smokey grey and yellow so far...
They're displayed in groups, by colour, and are the only colours in a fairly neutral house. Only one colour glass per room though. Green in the bathroom, pink/purple in my room, smoky grey in the kitchen, turquoise in the living room, orange in the hall. Yellow hasn't got a home yet...
I dream of having a huge window (in my fantasy loft apartment) where they could all be displayed as a rainbow of glass on glass shelves, with the light shining through.
And notebooks! Cos who doesn't love a beautiful, new notebook. This is accidental collecting though as I can't bear writing in them, it spoils their newness! So I buy another one, intending to write in it, then don't want to spoil it, so I buy another one and on and on it goes!