With complement: here is, are, etc. Without complement: here it is!.
Origin
Early 19th century; earliest use found in Percy Bysshe Shelley (1792–1822), poet. From French voici from Old French veiz ci from veiz, 2nd singular present indicative of veoir to see + ci here (from post-classical Latin ecce hic from classical Latin ecce behold! + hīc here: see hic jacet).