A number of online valuation calculators are available and they are as reliable as any of the printed price guides. The online valuations use much of the same information as other guides to work out your price.
Their big advantage is that you can tailor the information you supply more accurately than is possible with a printed guide - and the website will work out the price for you.
It's still best to check out other sources to gain a more rounded idea of your car's value - a glance over the classifieds will do.