Tamron 28-75 f2.8 Xr Di.
Putting a price limit and choosing a zoom lens... you really limit your choices.
You don't really talk about what you shoot, how you shoot, what you appreciate more than anything else.
There are a lot of zooms out there that are cheaper than $500. Most of them have some serious flaws. Lenses are a compromise.
Cost, weight, sharpness, fast AF, good color, distortion, chromatic aberration, zoom range, sharpness at full zoom, sharpness wide open, sharpness at all f-stops, aperture range, bokeh. I'm sure there are other things I'm forgetting.
If you take away the zoom, there are PLENTY of great lenses for under $500. 50 f1.4. 85 f1.8. I think the 100 macro. I think maybe the 17-40 f4 L is right in that price range...
Most cheap lenses have a hard time with sharpness, especially wide open. Most zooms have a tough time at full zoom too. When people post sample shots, make sure they're not at f8, and at the wide angle end of the zoom. 70-300's, and 18-200's, and other consumer zooms will have terrible image quality at various settings.
Read the lens reviews at FredMiranda.com. lens recommendations here can run all over the spectrum. One person's junk is another person's diamond... and with the sheer quantity of newbies here responding with, "Dude, get the 17-85, it ROCKS!"... they only care about zoom range and little else.
The Tamron's weakness is AF speed. If you're going to chase 10 years old around a baseball diamond/soccer field... you better know where the action is, and focus ahead of time... cuz the Tamron is not a "sports" lens. Canon's 24-70 f2.8 L is your lens if you want a sports lens. better still, the 70-200 f2.8 IS is the king of all sports lenses... but then it's $1650 mail order.
Good luck.
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'In 1983, the game of golf had a firm grip on the waist of my boxers and was administering the death wedgie. I had a dose of the atomic yips and after missing 10 of 11 cuts by a single shot, I was ready to quit and apply for a job as a wringer-outer for a one-armed window cleaner.'