Schlongberg Sachs is a company which engages corporate and investment banking. They are based on the real life financial institution Goldman Sachs. They are sometimes mentioned on Weazel News in financial reports.
The Schlongberg Sachs building is located on Asahara Rd., near Babbage Drive, in Alderney City, Alderney in Grand Theft Auto IV. It is a parody of the Military Park Building, also known as the NorCrown Bank Building, in Newark, New Jersey.
They are apparently very well known for their successes and their employees' high bonuses, referencing the lucrative bonuses usually given to bankers at the end of the financial year. This is supported by the fact when Playboy X tells Niko Bellic during Blow Your Cover "I pay like a Schlongberg Sachs bonus!".