PageMapper is a bookmarklet that shows you a map of places mentioned in the HTML page you're currently viewing.
To take PageMapper with you, just drag this link to your browser toolbar:
Clicking the bookmarklet adds a map to the upper right corner of your page. This map will have markers for locations mentioned in the page. Clicking the markers will open a popup, which displays the location name and a snippet of the text surrounding the location of the text in the page. Clicking the bookmarklet again removes the map.
Try it on this page: since this pages mentions Boston, you'll get a map with a marker on Boston. Make sure to drag it to your toolbar, so you can use it on other pages.