One modifies YouTube to allow you to keep track of which videos you've watched off of a user's /videos page, which includes when you search a user's videos on their profile. It doesn't do anything to an individual video page, or anywhere else the video might show up like in the recommended bar or main page search results, but I might add those features in future if I feel it's necessary.
The other modifies Creepypasta.com which is the source for short scary stories on the internet, but unfortunately has a pretty terrible website. At first the script did a similar thing that my YouTube script did, just keeps track of which pastas I've read, but now it also does a bit of styling help as well to make things look nicer and be more readable. In the past I would use the Chrome extension Readability where I hit the backtick ` key and the page magically transforms into a more readable text format, but now I don't have to do that unless I want to read on my Kindle (using the Send to Kindle extension and pressing alt+K).
The install process is extremely easy, just install the appropriate extension, Greasemonkey for Firefox or Tampermonkey for Chrome. Then on the "Info" tab of the userscript you want to install, click "Install this script". Greasyfork seems to do a pretty good job preventing malicious scripts from being uploaded, so that's a plus.