That would be the kidneys. The kidneys are part of the urinary system, and filter waste from the blood, and excrete them, with water, as urine.
In normal human adults, the kidney is around 11 cm long and 5 cm, and 150 grams. Kidney's weigh around 0.5% of a human's total body weight.
A variety of waste products are excreted from the kidneys, produced by metabolism, including the wastes urea and uric acid. Also, the kidneys secrete a variety of hormones, which include erythropoietin, urodilatin, and vitamin D.
Diseases and disorders of the kidneys include:
Congenital obstruction of urinary tract
Polycystic kidney disease
Unilateral small kidney
Humans can generally live with just one kidney, due to the fact that we have more functioning renal tissue than we need to live. This could possibly be due to the nature of the prehistoric human diet.