Amazon Link Globalizer
This plugin forwards Amazon links (affiliate or not) to the Amazon that is right for the user (e.g. amazon.co.uk for British visitors).
The plugin will install an output filter (onAfterRender) that checks for links to Amazon.com and Amzn.com. It then rewrites those links to point to the A-FWD service. A-FWD will do the country lookup (using Geo-IP) for you. You can specify your Affiliate IDs for all the country sites in the plugin's settings.