Bookmarklets!
We all know what bookmarks are - they're little links in your bookmarks bar, so when you click on them, they navigate the browser to the page which the bookmark link points to.
However, instead of a link, a bookmark can hold some javascript code instead, so that when you click on it, it runs the code on the page you're currently viewing, and thus affects the page content in some way. This is called a Bookmarklet.
[EDIT] Due to a newish security feature called Content Security Policy, some web-browsers erronously stop bookmarklets working on some pages. If these do not work, this could be the problem.
In order to put these bookmarklets into your bookmarks toolbar, do not click on the link! Instead, you should drag the link from this page onto the toolbar. Or, you can right-click the link and bookmark it that way.
Privacy Search
There are several search engines which are more privacy-focused than Google. Two well known ones are Brave and DuckDuckGo. I tend to use DuckDuckGo. Unfortunately, I sometimes find the results from these sites are not exactly what I'm looking for, so I have to try searching with both of them. The following bookmarklet will try the same search in the other search engine, so if you're looking at a page of Brave search results, clicking on it will run the same search in DuckDuckGo, and vice-versa.
(There is another search engine which is privacy focused: Startpage, but that handles its results differently.)
Move search to Google
Sometimes the results from DuckDuckGo or Brave are not what I'm looking for, and I want to run the same search on Google. The following bookmarklet will take you from a search result page on either Brave or DuckDuckGo (or Bing) to a Google search for the same terms.
Ebay
Sometimes I want to post a link to an Ebay auction on social media, but if you just copy and paste the URL, it often contains lots of un-needed extra text, creating an un-necessarily long and ugly link. This bookmarklet will give you a clean link to the currently-viewed Ebay item with the minimum needed information in it.
Internet Archive
Sometimes I want to look a webpage up in the Internet Archive (the "Wayback Machine"), either because it has disappeared, or I want to see what it used to look like. This bookmarklet will take you to the Internet Archive version of the page you're currently viewing.
Mapping Functions
I use maps a lot, and so I've produced a lot of bookmarklets to help me navigate mapping sites, or other geographicly related webpages. And now I'm gifting them to you!
Postcode Links
This bookmarklet will convert any UK postcodes on the currently-viewed webpage into links to Google Maps.
Selection to Google Maps or OpenStreetMap
If you select some text in the current page, such as an address or postcode, then these bookmarklets will search for that text on the relevant mapping service.