An interesting one is to slice the image using energy-measurements. The idea is to separate letters. You subdivide the image by having lines "move" across it.
Every line is like a "lightning strike" in the sense that it follows the path of least resistance. There's a general direction, for instance top to bottom. You draw it by choosing moving a point on the top of the image, and you go down. Every step downward it either goes straight down, or one pixel down and one to the left, or one pixel down and one to the right. It picks the step with the least energy, which means minimum color change. You can use straight euclidean distance, and you'll be amazed how well this deals with things like gradient backgrounds, but it works even better to use information metrics. Another slight improvement can be had by adding a small cost to horizontal offsets, so the lines go more-or-less, but not quite, straight down.
You keep subdividing the image, and then subdividing the subdivisions until you have to accept quite big jumps in energy, while enforcing a minimum subdivision size. This should "mostly" give you individual letters and those are easy to OCR.