Lately I've been getting enthused about a couple of Firefox plugins. They're both raw but I think they both point to the way forward for the programmable web.
Ubiquity is basically a command line for the web - a way to apply "verbs" to user input and web pages. Sample usage (and built in commands) include a "map" command to quickly build a googlemap and insert it into, say, the html in your webmail interface. What's cooler is that it provides infrastructure to quickly build additions to the browser that before would have to be packaged as bookmarklets or Firefox extensions. In the case of extensions it's frequently not worth the trouble and bookmarklets provide a lousy UI and development experience.
Ubiquity comes with a built in editor and commands can be tried out live in the editor or installed without any restarting required. To be honest the ease of writing commands made me think of emacs - executing code in the same tool you're writing it without any sort of compile->execute cycle. I even quickly whipped up an unwrap command when I was repetitively pasting sql statements from mysqlbinlog in a console to phpMyAdmin to restore some damaged data. Newlines inserted by the console were messing up the sql statements and visually scanning each statement was error prone. 30 seconds after thinking about it I had a two line Ubiquity command up in the editor that allows me to select some text in a textbox and unwrap it...