When you save a normal bookmark to your bookmarks toolbar or menu, it creates a link so that when you click on it it takes you to the linked-to page. However, instead of a link it can hold some code instead, so that when you click on it, it runs the code on the page you are currently on, thus affecting 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.
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 link to the currently-viewed item with the minimum needed information in it.
Remember, do not click on this link! Drag it into the bookmarks toolbar (or menu).
I use maps a lot, and I like to use different mapping sites for different things - Google for general browsing and Streetview, Bing for birdseye view, Streetmap for OS maps, etc. Unfortunately, if you are looking at something on one mapping site, and want to view it on a different site, it can be difficult to quickly go to exactly the same map location.
These bookmarklets help by scanning the current page's URL for valid latitude and longitude values, and using them to open a different mapping service in a new window.
These will work from Google Maps or Wikimapia or any other site which dynamically updates its URL with latitude and longitude values. Some other sites allow you to update the URL manually. Sadly, Bing doesn't do either.
Some other sites have location-based services as well...
This bookmarklet will convert any postcodes on the currently-viewed webpage into links to Google Maps.
On-Page Links to mapping sites
Sometimes you might find a webpage which has links to places on a mapping website, but you'd prefer if they were linking to a different mapping website. The following bookmarklets will try and change all the links containing latitude and longitude values on the current page. It also emboldens the text of any links it changes, so you can see they've changed.
I like to use Old-Maps.co.uk to see historic map information, but I find the search function difficult to use. So I created a bookmarklet so that I can find a location using Streetmap.co.uk, and then click the bookmarklet and it takes you to the same place in old-maps.co.uk.
Specific Location-based sites
Sometimes I'll be using a website, such as a property site or tourism site, which has location-based information in it, and I want to see the location in Google Maps. So I created these bookmarklets.
This first script finds the first instance of what looks like latitude and longitude data in a page's source code, and opens a Google Map centered there. It's a bit crude, but if a page only refers to one location, it often works. Also, this script specifically looks for coordinates in the UK first, to reduce the number of false positives. I created this bookmarklet specifically to work with Rightmove (while looking at a specific property), but it should work on other sites as well.
Some sites I use, the generic script doesn't work, so I've written specific ones:
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 current page.
Virgin Media Broadband
I get internet access from Virgin Media Broadband. If you go to a website which no longer exists, you are sometimes redirected to a page on the Virgin site which tells you that the page you want doesn't exist. Quite often I want to look in the Internet Archive, to see what was on the site before it disappeared, but because I got redirected, the URL of the page is difficult to access without going back to the linking page.
So I wrote this bookmarklet, which retrieves the URL, and then redirects you to the appropriate page at the internet archive.