Style differently the active (just clicked) link in Categories and other widgets

Is there a way to style differently the "active" (just clicked) link on WP's "Categories" / "Recent Posts" / "Recent Comments" / "Blogroll" and other rather standard widgets displaying collections of links?

Consider http://wordpress.org/showcase/tag/people/ as a prototype of the desired effect: under the "Popular Tags" in the left column, the link "People" has presently been clicked. Note that "People" is gray and is not clickable. It looks like this:

Popular Tags
CMS (83)
People
Technology (60)

Then, click the blue "Technology (60)" link: it leads to http://wordpress.org/showcase/tag/technology/. Now, "Technology" becomes gray non-clickable, and the link "People (64)" became clickable again:

Popular Tags
CMS (83)
People (64)
Technology