The sleeper sofa is a classic small space solution. For a roundup of resources, see AT's Best Sleeper Sofas 2008.
This Small Space Before and After includes lots of tricks for a tiny room including ceilings painted the same color as the walls, curtains hung beyond the window frame to extend the sense of space, and double-duty furniture.
If you don't have room for a sleeper sofa, how about a sleeper chair? Crate and Barrel has a good selection of twin sleeper armchairs.
Take a cue from hotel style and use a Folding Luggage Rack to give guests a spot for their suitcase. This image comes from the Bellinter House Hotel in Ireland.
If you're lacking a guest closet, use wall hooks or a wall-mounted clothing rack to make some extra space for hangers.
This Ottoman Sleeper from BoConcept triples as a slipcovered cube, a twin bed, or a cushioned bench.
Another classic small-space option: the Murphy Bed. For stylish examples, check out AT's Best of Modern Murphy Beds.
If you have room for an extra bed, it may as well double as storage space. For resources, see AT's Top 10 Storage Beds.
This post on How To: Make a Couch, Bed, and a Guest Room Fit into a Small Space provides some great DIY ideas for creating your own daybed.
If you need more ideas, hotels are always a great resource. For inspiration from the AT archives, check out Small Space Ideas from Room Mate Hotels and this Clip File of Inspiration from 9 Hotels.