The following website's can help you find reputable experts who are willing to provide you with free auction and sales estimates for many antique and collectible items.
Experts provide this service free (for items they sell) because they're hopeful you'll choose to consign your item with them. This earns a commission for their auction house or gallery.
Because accurate estimates require highly specialized knowledge (especially if an item's particularly rare) it's recommended that you get several estimates (at least 3) from different experts to compare your items valuation. Just as you would in any-other estimate situation (home remodel, car repair, etc).
Also, before consigning your items to anyone, see the Better Business Bureaus - Tips for Selling Through Consignment as a way to help insure that you're doing business with a reputable expert. (all links will open window)