Canon brand, under $500, short focal length. If you're looking for a zoom, that really narrows it down. The 28-135/3.5-5.6IS is the only one I can think of. There are two very cheap 75-300's, but that's a lot of overlap with your 100-400.

If you include primes, there are several - 20/2.8, 25/1.8, 28/2.8, 35/2, 50/2.5 macro, 50/1.4, 50/1.8, 85/1.8, 100/2.8 macro, 100/2.

If you splurge a bit, the 70-200/4L is about $625 with rebate right now. Much better quality than any of the above (it was my first lens almost 10 years ago), but still only starts at 70mm.