One new aspect of UserJS comes from the new Opera Unite application, UJS Manager. After giving UJS Manager control of your UserJS directory, it will show you a list of your scripts in which you can enable or disable the scripts. If the script is specially formatted for UJS Manager, you can even edit some variables in the script.
Another feature of UJS Manager that I find very useful is script installation. If you open a script file in opera, UJS Manager will insert a bar at the top of the page with a button to install the script. Once setup, UJS Manager makes Opera's UserJS experience much more easier and friendlier.
Now that I got UJS Manager out of the way, here are the scripts I currently use regularly.
Shrink large images to fit in the window.
Show the original post when hovering over reply references in 4chan.org-style sites.
Google Reader - Compact, Clean & Tweaked
A user style sheet for Google Reader turned into a script so it doesn't apply to all Google sites. It gets rid of all the extraneous parts in Google Reader.
No Click to Activate (direct link)
Does some magic so you don't have to click first on every flash video (and other embedded objects).
Open in background with long press
Click on link and hold to open the link in a new background tab. I use this with Google Reader and a few other sites where I want to open several pages to read later.
Shameless self-promotion on this one. Renders ruby annotation, popular in websites for some subsets of Japanese culture. Might also be useful if you're studying Japanese.
Another script of mine that enables Opera's FastForward feature for pages with a serial url, like /images/1, /images/2, etc. With this, you can just press the space bar to advance to the next page.
Adds links to download Youtube videos in FLV and MP4 formats.