Plastic file boxes are your friend!

Our apartment is tiny, there's just no room for a filing cabinet. As things stand now, my iMac is set up on a tiny old fashioned desk (built before manilla folders, I think).
We have one (1) built in desk with no drawers, but cabinets above it. In its defense, it is a huge desk, 5 feet, so there's a decent amount of room. My husband gets this desk, and we store the file boxes underneath it. One box is special, it's the 'Current' box - it has our tickler file, the files for the current years bills, and anything else we use regularly. It has a compartment in it's top, where we keep spare pens, deposit slips, white out, paperclips, etc.

It's not ideal. But it works.