Eze-Lap model S works well for me, $6.95
IIRC it is 600 grit diamond and also has a hook groove.

I got a Lansky Quick Edge for Mother's Day. I've used it to touch up my two fixed blades, which we beat the snot out of last year, and it worked OK. It's bigger than my little Gerber sharpener but it's still small, light weight and inexpensive.

Edit: "small" not "smallpox".'':-)

I prefer sandpaper and a leather belt. Light, easy, effective.

i keep a Speedy Sharp handy for quick work on cheap knives.

fast, easy, sharp, but not a fine pretty edge.

http://www.amazon.com/Speedy-Sharp-Orange/dp/B002OG3SKC

I would go for a Fallkniven DC3
It will even sharpen hardened steel like ZDP 189 and Tool Steel like D2

I'm using this one: http://www.ebay.com/itm/251813416755 in my knife's handle (see the 3D printing thread) with the pen shell removed
Ultra compact! This one http://www.ebay.com/itm/262137581988 is my pocket EDC. They are good enough for maintaining the edge as well as for fixing it if broken. It's essentially a diamond file which has much more uses than just a knife sharpener (especially the conic one). Some knife sharpening skills are strongly advised, it's not a self-guiding type.

If you browse eBay you can find them even cheaper.

I also use a simple stone that I used to use to fix burrs on my hockey skates. Most sporting good stores around here carry them for only a few bucks. Of course, it's been a long time since I laced up those skates, so the cost has probably tripled. LOL!