jBidwatcher is a Java-based application allowing you to monitor auctions you're not part of, submit bids, snipe (bid at the last moment), and otherwise track your auction-site experience. It includes MANY currencies (yen, pound, dollar [US, Canada, Australian, and New Taiwanese], Swiss Francs, and euro, presently), drag-and-drop of auction URLs, a unique and powerful 'multisniping' feature, a relatively nice UI, and is known to work cleanly under Linux, Windows, Solaris, and Mac OS X from the same binary.
Currently it is only implemented for eBay, but it is abstracted suitably to work with any online auction site with a minimum of changes.
It is under active development, and issues are quickly responded to. Bug reports and feature requests are always welcome, as are praise and complaints.