The Krogan Monument is a huge statue of a krogan warrior in the lake near the Citadel Tower on the Presidium. It was commissioned to commemorate the krogan contributions in the war against the rachni. It was the subject of much controversy after the Krogan Rebellions because some people thought it should be removed, but the Council decided that the role of the krogan in saving Citadel space was a historical fact that should not be forgotten.
Wrex likes the Krogan Monument; it seems much more impressive to him than the Relay Monument. If Commander Shepard speaks to him in front of the Monument, Wrex soberly says the statue represents the values of the krogan - honor, loyalty, courage, fortitude - which have now been lost to his people since the Krogan Rebellions and the devastating genophage. Either human squad member and Liara will comment that Wrex really isn't like other krogan; Wrex shrugs and says "I'll take that as a compliment".
However, Tali gets annoyed around the Monument because even though the krogan turned against the other Citadel races and almost conquered them, they get a statue on the Presidium while the quarians are forced to live in exile. Wrex points out that the quarians are still alive, while in two hundred years or so, the Krogan Monument might be all that's left of his people.
Provided Grunt lives and is recuperating in the nearby Huerta Memorial Hospital, he and a couple of his Aralakh Company squadmates pay the Krogan Monument a visit in 2186 while on drunken revelry, clambering over it instead of simply staring at it. They manage to take pictures of themselves on the memorial before carjacking the C-Sec officers who arrive investigating the public disturbance they were causing.
In Mass Effect 2, Morinth has a scale replica of the monument in her apartment on Omega. She claims it was given to her by a suitor who eventually got what he wanted, only to find it wasn't what he expected.