I use a Speedy Sharp, it may not be the best but when you actually use the knife and need a quick sharp edge then they are hard to beat, and if you are careful you can even sharpen serrations. They are cheap and can fit just about anywhere. I have a few of them laying around in different places and I have never messed one up or wore it out. They are also good at deburring or scribing metal along with sharpening about anything else you can think of.
http://www.speedysharp.net/
http://www.speedysharp.net/