I've written a UK Postcode bookmarklet , I find it comes in quite handy, it works in both I.E and Firefox, havent tried it any other browsers. It basically lets you select a postcode on a webpage, or enter one at the prompt if you havent selected one, then takes you to the streetmap.co.uk map for that postcode.
Find a UK Map by selecting/entering a postcode, place name or road name Bookmarklet
You can test it out by selecting a postcode on a page for instance this one NW1 4BG then just click the link. If no postcode is selected it pops up a prompt for you to enter one. Sweet! To install it on your browser just rightclick on it and add it as a bookmark, or you can just drag it on to your browsers links toolbar.
Update: I just discovered that the bookmarklet also works with UK place names or road names, schweet!
Update 2:
Ive updated the script so it will work even if the page you want to get the postcode from is inside a frameset