Dan Webb at The Web's Bollocks has posted a great article, Cleaner Callbacks With Partial Application. It describes ways to use JavaScript closures to keep your code cleaner and more reusable, especially when creating XMLHTTPRequest callbacks. If you understand the basics of closures but want to see some practical applications, go check it out.

If you're new to closures or have been struggling to understand how they work, I recommend reading Mark Wubben's introduction, Getting Funky With Scopes and Closures. For a more detailed and thorough analysis, be sure to read Richard Cornford's JavaScript Closures.