I am quite fond of the Invisible Hand addon for Firefox. It’s one of the few addons I’ve kept installed through browser upgrades and machine migrations.
Install it and it lurks in the background until you view a web page listing a specific item for sale. When that happens, it pops a little one-line banner up at the top of the page letting you know whether you’re getting the best price available on the item or not. It uses pricing information from a selection of retail sites, but as far as I can tell, it doesn’t do currency conversions, and it definitely doesn’t factor in any potential shipping charges, so you’ll need to ferret out that information for yourself.
If there’s a better price than the one quoted on the current web page, it will let you know what the best price available is and where to find it. If you’ve already found the best deal, it’ll let you know which retailers come close.
At the time of writing, the addon description says it uses data from 580 retailers across the US, UK and Germany. For those of us in Ireland, it’s always worth checking the local retail sites too as shipping can sometimes be extortionate to our fair isle, negating the usefulness of any product pricing bargains.
In the few years I’ve been using it, the addon has saved me a few bob here and there, and occassionally provided useful pointers to unfamiliar sites with good deals. Give it a go.