Using the parameters below you can grow your own trees using fractals (well, approximately a fractal). The tree is generated by starting with a trunk of a certain length and then adding two branches that split off at a specified angle and length that is a ratio of the trunk. We continue adding these split branches for every branch that is drawn, up to a certain depth. If you were to repeat this process, as the limit approached inifity, you would have a set of numbers that were of a fractional dimension and had a self-repeating structure. Namely, a fractal set.
I advise keeping tree depth under 20. The computations grow O(2^n) so your browser can crash if you push it. By about 15 you'll notice a serious slowdown. The length ratios are really nice in the .6-.8 range, and quickly become impossible to render for values greater than 1 (if you do want to go big, I'd take my code and re-render it with a bigger canvas). Width of trunk looks pretty natural between 10-20 (pixels) and the height 100-300. The colors need to be specified in hex (e.g., "#FF0000" = red). If you need help finding colors you can try this or this. I recently also added the ability to add some randomness into your branch angles (+/- 10 degrees).
Start off by hitting submit to see the basic tree then start playing around and see what you find. You may need to scroll down to see the tree because I had to use a pretty large canvas to support the range of tree sizes. While the suggestions above are starters, you'll find some beautiful behavior in other ranges. Have fun!