http://www.defensedevices.com/dolopo.htmlI've seen these portable door locks in catalogs, but never used one. It seems like a good design for stopping someone from using a key to defeating the keyed lock to get in. It would only be as strong as the frame and the door though if someone tried to to kick the door in.
I've seen devices that are a bar that goes from the door knob to the floor at approximately a 45 degree angle. I don't think I would not trust one of those to dig into the floor enough to do much.
There are various portable alarms also, but I question they're real usefulness. I suppose the noise might scare off some bad guys.
BTW, I'm not endorsing that site I linked to. I don't know anything about them, but they had a good picture of that lock.