Tamron has alot of great options for low cost lenses under $500 even brand new. For Example you can get a 17-50 f/2.8 without VC for $499. However you can also find this lens used on ebay for about $200. You can even find the VC version for $300 on ebay, which is normally $650. Tamron also has a 28-75mm f/2.8 fetching about $300 on ebay. Tamron also has a 70-200 f/2.8 (non-image stabilized) for around $750 new that usually sells for about $500 used. Just as a warning though... Tamron Lens tend to break easier than other lens manufactures. (I've broke one myself)
Canon L Glass
Yep you read that right. You can get Canon L Glass for about $500. Which models? The 24-105 F/4 IS is a great choice for an upgrade to that hideously awful 18-135mm kit lens. You can also find the Canon 28-70 f/2.8 for somewhere around the mid-$500s. In that same price range is the Canon 70-200 F/4 (non-image stabilized). Although not a zoom it's worth mentioning that the Canon 200 F/2.8 is also about $600 on ebay.
If you need wide angles, go with Tokina. Two great ones are the Tokina 11-16 F/2.8 and the Tokina 12-24 F/4.
Zachary Will is an Award-Winning Filmmaker creating short films and content for the Internet as the owner of Riverside Studios. He has a Bachelor's in Film from Full Sail University. He has worked on productions for companies like Webmd, Sears, Home Depot, Six Flags and more!
"the trick is to never give up... I think the secret of success is not how little you get knocked down but how often you get back up." - Gareth Edwards