Bookmarklets are the handiest little things. In case you don't know (which I'm sure you do), they're small chunks of JS that you store in your browser's Bookmarks or Favourites section which you can launch while looking at any web page. I write bookmarklets for all kinds of different tasks – navigating quickly around the build monitor at work, filling in tricky forms that my browser autocomplete doesn't handle, etc.
Here are a few of bookmarklets I find extremely useful for web development. To add them to your browser, simply drag the link on the title into your bookmark bar, hold shift down and drag or right-click and 'Add to Favourites', depending on what browser you're in.
This will allow you to add any script you like to the current page. This can be particularly useful if you want to add a certain library or plugin to a page to investigate it further.
A variation on the above bookmarklet, this simply adds the latest version of jQuery. If you want to be able to play with a page and are familiar with jQuery, this will ensure that it is loaded and attached.
Add any stylesheet to the current page with a particular URL. This is handy if you want to demo variations to clients, I find. particularly if you predefine the CSS URL.
This isn't so much a web dev helper, more a general helper. I use this (or a variation thereof) to submit posts to specific reddits with fields prefilled. This bookmarklet defaults to the webdev subreddit.
I can't remember whether I wrote this one or if I found it somewhere. The 'x<style>' is something from the middle of jQuery, though. Anyhow.
This allows you to add a style block directly into the page. This is useful for small CSS injections where the browser's web inspector is unavailable or incapable. Even though it's a single prompt that pops up, there's no reason why you can't past an entire stylesheet in there. I sometimes find myself doing that when I'm previewing designs on a production server.