I've made Simon Willison's blog one of my daily reads, and was reading some of the archives in the CSS category when I chanced upon a 1-month old entry that was an absolute gem.

To be honest, I didn't previously bother with bookmarklets because I thought they were cool, but lacking in practicality. Until I saw this...

With this, you can experiment with the CSS styles of your website(s) for those 'what if?' moments...

I've replicated the 'edit styles' bookmarklet here. Just click on it, or drag it to your bookmark toolbar if you think it's useful (don't think too much - it IS).
» edit styles bookmarklet

With this, you can experiment with the CSS styles of your website(s) for those 'what if?' moments. You can also change how a site looks to your liking (too small font sizes, glaring background colors) at the click of a bookmarklet and some CSS editing.

For more useful bookmarklets, visit Jesse's Bookmarklets site, in particular the Web Development Bookmarklets section.