I have 3 small filters spare under the bed along with a 3 gallon tank ( that I'm not allowed to use by the law of mother's wrath ) I have a UGF that came with a second hand 5 gal tank , a small HOB and a smallworld mini filter still in the packaging ( It was only £3.99 and I have an airpump already if I need to use it!!! )
I had a spare Fluval 1 in my 30 gal just sitting around cycling for no reason...until I discovered one of my female bettas was in fact a Plakat male, at which point out came the spare 5 gallon, and in went the Fluval 1 plus sand, plants and Bronson.
You never know when you'll need a spare filter! I think I may get a big model of the new Fluval U internal series to sit in my 30 gal for a bit of extra filtration actually....