US-centric:Firefox only (Windows/Mac/Linux): Search one site for a product and get comparison prices inline from several other merchants with the PriceAdvance Firefox extension. PriceAdvance works very similarly to previously mentioned extension Book Burro, but it works with products beyond books. Currently PriceAdvance supports sites like Amazon, Best Buy, Circtuit City, and Wal-Mart, and it's likely to add more as it progresses. You can disable PriceAdvance when you're not shopping through the status bar so that it's only comparing prices when you want it to. Looks like it could be a good addition to your online power-shopper toolbox. PriceAdvance is free, works wherever Firefox does.