Are there any out there? I would really like to read a spy manga. And not a shoujo spy manga. I want action, stealth, conspiracy, plot twists etc. It doesn't have to be about secret agents just something in that same vein. Like mercernaries or hitmen or something with an agency and people with missions. Now I'm rambling. 😛 Well, thanks in advance!

sanctuary might fit a little

[Life is But A Dream..]

Some can fit only partially but they are all good manga:

Ares mercenaries, there's a spy.
Gunslinger Girl hitmen, there's an agency (my fav manga)
Shin Angyo Onshi it has action, conspiracy, plot twists.
Vampire Juujikai it has action, conspiracy, plot twists.
Kurozuka it has action, conspiracy, plot twists.
Tista hitmen, agency.
Spiral it's a mystery manga, it partially fits.
Eden
Full Metal Panic kind of.

What I like in Trivial Pursuit style. Pick your category:[img]http://i379.photobucket.com/albums/oo233/Reid4891/Manga/trivia-1.jpg[/img]

From Eroica with Love fits perfectly. Lots of espionage actions during and after Cold War era (complete with NATO, KGB, CIA, and other intelligence agencies from other countries). Better description and some previews at here.
Yep, it's shoujo and has some shounen-ai elements (subtext only, btw), but it also has all of your requirements.

or you can read Der Freischutz and Z, the side stories, featuring NATO's agents on their missions.

[img]http://i75.photobucket.com/albums/i285/yuuri097/Hanashizume_no_Utage.gif[/img]

Dogs(Has an ex-hitman in it)
Dragon Hunter(Mercenary)
Nazo no Murasame-kun (Father is a spy.)
Peace Maker(Mercenaries as antagonists)
Samurai Champloo(Mercenary)
Vinland Saga(Mercenary)

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Jackals maybe? Not spys, but they are assassins you can hire.

Does the walker choose the path, or the path the walker?

Haha, I have lots of stuff with spies, but it's mostly yaoi, but I know of one other...

Project ARMS Sounds like almost exactly what you are looking for.

[img]http://kofk.de/C2C51774.png[/img]

I'm surprised no ones suggested CAT yet. It definitely fits what your looking for.

Also, you could maybe check out Transistor Venus. It's all about spies. The art style might be too simple for some, and it's not exactly serious or realistic either. It's got shoujo ai themes though...

I'll also second Gunslinger Girl for something about an agency and hitmen. It's an incredible manga.

Tree + Taco = Tree taco.

Hmm, I think people are straying from what OP wanted...
I mean, Vinland Saga has nothing to do with conspiracies, espionage etc.
I suggest;
Kaikei Saketen
Benkei In New York
Strain
Banana Fish
Lupin Sansei
Smuggler
Sousakusha
Crying Freeman
Pluto
Golgo 13