For inexpensive knives there are several options available from Wal-Mart, sporting good stores, and kitchen stores that will apply a decent edge.
For higher quality knives (i.e. the ones I wouldn't take to a BBQ competition) there are higher end systems such as the Lansky which I currently use. That system has some limitations which have led to a newer generation of systems including the Edge Pro and Wicked Edge. I found a good comparison of the two on YouTube a few months ago that I found very illuminating. My main take away was that the Wicked Edge system was "better" for the knives that it would sharpen, but that Edge Pro worked on more knives given their size and geometry. The Wicket Edge system is also more expensive. I have been planning to replace my Lansky with an Ede Pro for some time, but just haven't gotten around to pulling that trigger yet. The bigger question to me has been the correct brand and progression of stones to use with whichever system you choose, and the Edge Pro seems to offer more stones. This alone is what kept me from buying most recently. My biggest question was that if the dry diamond stones of the WE are so great, why isn’t there a comparable system available for the EP? If you do go with an Edge Pro system, I recommend you get and Angle Cube to compliment it.
There are also many people who insist that free hand sharpening is the way to go. This requires more manual precision, and still ends up asking the question of which progression of stones is the best. I have found that knife sharpening is a hobby and passion by itself, much like BBQ, and while the two overlap in some areas, if you spend too much time trying to get "the best," you'll miss out on having something.
dmp