There's little doubt that Margreet Zelle (Mata Hari's real name) was a spy of some form — but it's decidedly uncertain whether she ever provided either side with information of any real tactical, strategic or political value. In fact, she seems to have been a charismatic socialite and courtesan who cultivated relationships with prominent men (including various military officers, politicians, industrialists and aristocrats) across Europe, but was probably not able or even particularly inclined to cultivate these men as productive sources of intelligence.

During the First World War, she was recruited by the Deuxième Bureau, the French military intelligence service, to spy on Germany. As a citizen of a neutral power (Zelle was Dutch), she enjoyed relative freedom of movement around Europe.

It also appears, however, that Zelle probably volunteered her services to German military intelligence, making contact with German officers in Madrid and The Hague. British counterintelligence officers twice arrested and interrogated Zelle on suspicion of espionage, but failed to find any concrete evidence against her. MI5's file on Zelle makes for very interesting reading; it includes much back and forth between the British and French intelligence services about Zelle's wartime activities.

To my knowledge, there's little evidence of Zelle providing either side with much beyond cocktail party gossip; the general consensus now is that she was an opportunist who took money from whichever side was offering at the time. Her indiscreet double-dealing, discovered by Deuxième Bureau cryptographers in 1916 (and to which she later confessed under interrogation), resulted in her trial and execution as a double agent.

There have long been claims that she was unjustly railroaded to the firing squad, but few have argued that she was actually 'innocent', per se; more that her activities were simply inconsequential and that the scale of her espionage was grossly exaggerated for propaganda purposes. Her sexual adventures, exotic persona and tragic end certainly provided fertile ground for a great legend to grow up around her. But whatever the truth of her spying career, she was ultimately a victim of the climate of the time: in wartime, even ineffective spies tend to receive short shrift when caught.