The bookmarklet (or extension, plugin, add-on, applet) when launched
would allow you to select an element on the web page you're looking at
by clicking (shift-click for multiple) and show a selection panel
which contained font listings from a number of web font providers
(Google, Typekit, etc). Selecting a font would then add the styles to
the page to enable that font on those elements. Regularly/recently
used fonts could be a quick selection in the panel.
A bookmarklet that allows you to highlight an element on the page then select and substitute a webfont dynamically
This idea is liked by 3 people