Oddly enough, the main thing I use to sharpen my knives is the angled stone on the back of my electric can opener. I gave it a shot a while back when I was thinking of having my knives professionally sharpened and it worked great.
I primarily use a steel, but of course this is just for keeping the edge of the blade straight - you're not actually sharpening the blade that way. To sharpen it you need some kind of stone or other abrasive. If you have a can opener, silly as it sounds, I'd try the stone on the back of that first.
Otherwise, actual sharpening stones aren't that expensive. I've been meaning to get one for quite a while, but the can opener keeps putting that purchase off.
And to give you an idea how sharp I'm able to keep my knives with that can opener & steel, a week or two ago I had a guy come to the house to give me a Cutco demonstration. My knives, including my 25-year-old wedding present Chicago Cutlery knives, were as sharp as his factory-edge knives out of the box. He was shocked that I did all his cutting tests with both my Henckels knives and my old crappy knives, including cutting his rope and cutting his gnarly old leather with ease.