The userboxes below are the ones I've made myself. Like everything else in my userspace you should feel free to use, edit, improve, borrow, steal and take to another wiki. Basically — do what you wish. If you do anything non-trivial I'd appreciate it if you let me know — but I won't complain if you don't.
Sister wiki edit
A userbox to point to your userpage on another Wikipedia. I found plenty of userboxes that would do this for commons, simple, wikinews etc, but none that would point to a different language's Wikipedia. My linguistic skills are non-existent so I was a little surprised that I was the first editor who found this to be a problem...!
This userbox might also be useful if you're wanting to create your own non-trivial userbox — it uses some conditional logic and has separate documentation so might be a useful base for copying into your own userspace and experimenting.
{{User:TFOWR/Userboxes/Wikipedia/Sister Wiki}}
 | This user has a page here. |
Not very useful — particularly if used on your userpage 
{{User:TFOWR/Userboxes/Wikipedia/Sister Wiki|de}}
 | This user has a page on de.wiki. |
Probably the most useful variant for editors with the same username across all projects.
{{User:TFOWR/Userboxes/Wikipedia/Sister Wiki|fr|Utilisateur:This flag once was red}}
 | This user has a page on fr.wiki. |
Useful if you use an alternative username on another project.
Barnwale of Epic Fail edit
Usage edit
{{User:TFOWR/Userboxes/Awards/Barnwhale of Epic Fail}}
{{User:TFOWR/Userboxes/Awards/Barnwhale of Epic Fail|2}}
| User:Giftiger wunsch/Failwhale | This editor has been awarded the Barnwhale of Epic Fail 2 times |
Order of the Mop edit
Usage edit
{{User:TFOWR/Awards/Order of the Mop}}
 | This admin has been awarded the Order of the Mop |  |
Zen Garden Award edit
Usage edit
{{User:TFOWR/Userboxes/Awards/Zen Garden Award}}
{{User:TFOWR/Userboxes/Awards/Zen Garden Award|2}}
 | This editor has been awarded the Zen Garden Award for Infinite Patience 2 times |
Expatriate New Zealanders edit
Like many Kiwis, I live outside New Zealand. All the Kiwi userboxes I found added me to the Wikipedians in New Zealand category, which isn't appropriate for us Kiwi exiles. This userbox aims to fix that. I shamelessly stole it from another editor.
{{User:TFOWR/Userboxes/Personal/Kiwi}}
 | This user is a Kiwi. |
Freedom Press edit
Probably not hugely useful for the vast majority of Wikipedians, but it's a useful way to say "I am an anarchist" without annoying non-anarchists too much...
{{User:TFOWR/Userboxes/Personal/Freedom Press}}
 | This user supports FREEDOM. |
Red Clydeside edit
Again, probably not hugely useful for the vast majority of Wikipedians, but I thought it might be useful to let editors know where I live. I'm not a big fan of flags or nationalism (or even nations...) and wanted to avoid The Saltire on my userpage. (It's a beautiful flag, but it's still a flag...)
{{User:TFOWR/Userboxes/Personal/Red Clydeside}}
| This user lives in Red Clydeside. The ficht fe justice cannae wait! |