Arduinna

These are Greasemonkey scripts, which will work in Firefox and I think Chrome (and Chrome clones) and Opera. If you don't have Greasemonkey, it's definitely worth installing. You can get it in add-ons/extensions. Once you have the add-on, you can install all kinds of scripts to customize your browser to do what you want.

DW scripts:

Dynamic and persistent DW reading/network page expand/collapse/hide This is amazing, and I wish I could install it on my mobile devices. It lets you collapse (and later expand) or completely hide entries on reading and network pages on DW -- not just your own, but anyone's. Collapsing a post will leave the subject line; hiding it will make it go poof.

LJ New Comment "This script remembers which comments you've seen on LiveJournal or Dreamwidth, and marks new comments with a link which says "NEW". Clicking the link selects that new comment."

Dreamwidth ljwho This lets you add notes to anyone's username on DW. It's browser-specific, so it's less useful than a native action tied to your account, but I still find it plenty useful.

You can find more DW scripts here.

AO3 scripts:

I found these through ao3some, a comm for AO3-related stuff.

ao3 download buttons This adds a Download button to everything listed on a works page, so you don't have to click into something specifically to download it. Just click the download button right there in the header info, choose your format, and you're done.

AO3 Kudos Tools Once it's set up, this will let you know if you've already kudosed something by changing the color of the kudos button to green, and putting a green background on your name in the kudos list. You can also set it up to put a dark grey background behind anyone else's name, if there are people whose tastes you really trust.

Fair warning on this one: it takes a bit of work to get set up -- not very much! But you need to edit the script once it's installed to add your username to the script so it knows what to look for, and to add anyone else's usernames if you want it to check for them, too. See below for instructions if you're not used to doing this.

AO3 Saved Filters This adds a section to the sidebar with two boxes, one for global tags and one for fandom tags, which will save whatever you put there. This took me a bit of figuring, but once I got it, I was floored.

Both of these will take either includes or excludes. To include something, just type it (enclose it in double quotes if it has more than one word: "hurt/comfort"). To exclude something, type it with a minus sign in front, again using double quotes if there's more than one word: -"hurt/comfort".

The "Global" box is persistent -- anything you put there will be filtered on no matter what works page you're looking at, and it remembers it across browser sessions. If you never want to see fluff, add -fluff to that box and hit save, and fluff will forevermore be filtered out of any results for you automatically. (You can always edit the boxes later to add or remove things.)

The "Fandom" box is for stuff specific to whatever fandom you're looking at, so you have to be on a specific fandom works page for it to show up (meta-tags don't count - "Sherlock Holmes - All Media Types" won't have a "fandoms" saved-filter box, but "Sherlock (BBC)" will. Collection works pages don't count, either. Both those pages will only have a global box.) This box is persistent for a given fandom: if you put -Lestrade into the box on the Sherlock BBC works page and hit save, you'll never see any works with Lestrade named in the header info when you're browsing through the fandom.

So if you never want to see fluff anywhere, put -fluff in the Global box and hit save. If you're good with fluff everywhere except The Wire, put -fluff in the Fandom box on The Wire's works page, and now that will be the only fandom where it doesn't show up.

I am just boggled at how customizable this is. Wow.

AO3 savior Works like Tumblr savior, in that anything it protects you from is hidden, rather than gone completely. You can choose to unhide them, then hide them again if you realize that nope, you really didn't want to see that after all. You can specify authors, tags, or summary terms/phrases; matches need to be exact.

Fair warning on this one as well: you also need to edit this script to tell it what you want it to protect you from. Again, it's not hard, but if you've never done it it can look confusing; instructions below.

( Editing those user scripts )

Couple of important notes:

1. Greasemonkey is installed on your browsers, and have nothing to do with your DW/LJ or AO3 accounts. You need to install it on every browser you use, and set up the scripts you want on each one.

2. This is much harder on mobile devices. iOS won't allow extensions to be installed unless you jailbreak your device, which I haven't done so I can't tell how well it works.. Looks like Android has a TamperMonkey app that's a browser that allows Greasemonkey scripts, but I don't have any Android devices so again have no idea how well it works. I can't find anything at all about installing Greasemeonkey on Windows phones/tablets on a fast search.

But if you're still regularly using a computer or two, these scripts are really useful.