I carry my Leatherman Super-tool on my belt everyday, and looks like a harmless multi-tool that most everyone is familiar with seeing.
It’s blade hides well until its needed, and locks in place when opened.
A Leatherman is useful and non-threatening… Especially the small ones.
On my keychain I carry a Tekut LK5050 “Dapper”, which is small and rainbow colored. It doesn’t look very threatening, and the carabiner-style clip is great for keychain use. It’s about the size of my car key.
Edit: Here’s the product on fasttech, now that I’m back to my computer:
http://www.fasttech.com/product/1556901-tekut-lk5050-dapper-stainless-steel-folding-knife
I carry one of these little guys every day. Very, very lightweight. Being mostly wood, it’s pretty unassuming. High-carbon blade can be made very sharp, though sorta brittle - which brings me to the biggest plus — they are CHEAP!
I edc this one and its pretty small and has other useful tools
non-threatening? Not sure - but I dont get the usual “WOW OMG what a knife! ” when I pull it out.
Spyderco Ladybug in a bright cheery color.
I have that dapper, yes, it is tiny and colored but it is veery pointy, i do not know if somebody can see it like threating.
This one looks very non-threatening! 
Just an FYI the right row above is actually not a ladybug, that’s the manbug model. Also both models offer very high end steel options and ever sprint runs (usually limited to 1200pcs limited runs).
For a larger option check out the boker plus Excalibur. A nice little cross of traditional (blade shape, nail knick) and modern (back lock* material)
Free at McDonalds.
I have that little toy, bought several years ago when Amazon had a sale and offered it for about $150 less than the current price. Spectacular but too wide to be very practical.
Hey, that’s my letter opener! I was wondering where that went…
(actually, these work surprisingly well for that purpose)
I made one of those into a spudger, it works surprisingly well.
How about this. 
LOL! How do you think they will react if they see this in your office…
How about a pair of those Gerber take apart vital scissors? I have a pair and love them. Super useful. Mine even came with a nice cheesy plastic holster with worthless sharpeners. 
Non-threatening knife?
One of these is quite non-threatening. The other is a serrated Cold Steel Vaquero XL.
:evil:
Wow, thanks for all the suggestions! I wasn’t expecting this much of a response.
I went ahead and ordered the SAK Centurion suggested earlier by CryusAM. I’m also slightly embarrassed to admit that I ordered the Tekut Dapper suggested by ToyKeeper for it’s…. rather unique look.
I also got a good chuckle from some of the (hopefully) less serious responses. @fartybum: I remember once I used to use a cheap plastic butter knife with the tip squared off as a thermal epoxy spreader for GPU RAM chips
@freman: Is that the Cold Steel Mini Tuff Lite? It looks great! I think I saw that very knife earlier today in another thread here on BLF.