After spending a weekend looking for the most striking example of Greasemonkey's power, I gave top honors to two scripts. (And an honorable mention to this goofy but eye-opening script that turns Boing Boing into an interactive quiz by replacing the author bylines with multiple-choice popup menus.)
Magic Line crawls every page you visit and creates a searchable history of links and RSS feeds you might have missed. Hold down Ctrl-Shift-L and a semitransparent search pane appears atop your browser window. I typed "New Orleans" and found a blog entry from the storm zone that I'd overlooked while speed-surfing.
The other winning script overhauls the Library of Congress'American Memory photo collection, a valuable archive hobbled by HTML pages straight out of 1995. Install the American Memory Fixer and the exhibit gets a makeover: tidy white pages with new fonts, a smarter layout, and large photos rather than thumbnails. It's more than a modification—it's a renovation.
These demos inspired me to write my own user script to streamline the site I've been working for into a lightweight search page. After an hour of fruitless keyboard-pounding, I realized the guys who wrote Magic Line and American Memory Fixer spend a lot more time on this than I do.
That's the limitation that threatens Greasemonkey's potential: Unless you're a natural programmer, you'll just have to hope some hacker shares your annoyance with, say, Slate's front page. So far the distribution of scripts is twofold—they're either for superpopular sites like CNN or programmer's hangouts like Slashdot, with a few niche favorites thrown in. (I haven't found a single script for Slate yet.) You also better hope the guy who programmed your script knows what he's doing. Buyer beware, because not all Greasemonkey scripts work flawlessly: My editor installed one that promised to trim the ads on IMDb, but it erased his whole page instead. (A script that doesn't work as advertised is annoying but won't cause any permanent damage—it's easy enough just to turn it off).
Greasemonkey recalls the giddy fun of when browsers first came out a dozen years ago. It also suffers from the same problem that hamstrung the Web before the arrival of easy-to-use site-building tools. Only a tech-savvy elite knows enough to play with it. The rest of us are still consumers. What we need is a Greasemonkey Remix Tool, a what-you-see-is-what-you-get editor that lets us redesign other people's Web pages on-screen, then spits out a script to make it happen. Something this powerful is too good to let the hackers have all to themselves.