The 70-300IS is a pretty good "all-around" lens...it's surprisingly sharp, light, unobstrusive, rather inexpensive, has a pretty long focal range, and IS can be very helpful. The 70-200/4L is another great all-arounder, but comes with the extras that an L-lens offers such as better build quality, slightly faster/quieter autofocus, and almost unparalleled reliability. Unfortunately, you're limited to 200mm at the longest, and it doesn't have IS.
Lenses like the Sigma 70-200/2.8 and 100-300/4, which are barely in your budget, are more specialty lenses. The 70-200 is big, heavy, expensive, and only 200mm at the longest. However, its fast f/2.8 aperture is invaluable when shooting in lower light or when trying to isolate the subject. The 100-300 is bigger, heavier, and even more expensive, but is one of the only lenses in its class and it does a great job doing what it's supposed to do. However, due to its weight and focal length, it would probably be best to use with a tripod most of the time since it doesn't have IS.
Those are pretty much your options. All of the lenses listed are great lenses, so weigh out your priorities and go with one that will suit you the best.