There are some companies, but I do not recommend them. I recommend looking at Craig's List and Rent.com. If you would like some help deciding on neighborhood, please contact me. Unfortunately, real estate broker's here do not handle rentals. If you would like to buy a home, or have any other questions feel free to contact me.
Best of luck.
There are several services here in Denver. try this one - http://www.DenverRental.com. They have a good website and seem to have nice properties available. I am sure you have heard that it is in most cases better to purchase than to rent. - http://j.mp/S2Hup4. If you would like to check out the market for sales or to get prequalified with a lender don't hesitate to contact me.
Robert McGuire ASR
Your Castle Real Estate
Direct - 303-669-1246
I can't say I know of any apartment-finding services, though there are some companies that manage a number of apartments. But I think most people find units by tracking new listings online at sites like Trulia. There are several good sites which you can access from the blog post below. The links are set for Boulder searches but can easily be reset to your target areas.
Best of luck finding a great apartment!