I’m on Chrome so some extensions may be Chrome only.
- Social Fixer - if you use Facebook and you don’t have this, I don’t understand how you use Facebook.
- Hover Zoom - Zooms a thumbnail image on mouseover on pretty much any site (you can disable it on any site too). On Tumblr this means you rarely have to click to enlarge an image.
- Flickr Photo Page Enhancer - does a bunch of stuff but for me the most useful is that it adds direct links to different sizes of a Flickr photo.
- Shut Up - disables comments on pretty much all sites — you can re-enable them by site though.
- Linkify Plus - automatically converts text URLs to clickable links.
- YouTube Options for Google Chrome - gives you control over YouTube — disable comments, automatically expand description, disable autoplay, set minimum size/HD level, disable annotations, etc.
All of these make my online life a zillion times easier! I hope they’ll do the same for you. This isn’t all the scripts/extensions I use, but they are the ones I figured would be most useful to the most people.