- "My little birds are everywhere."
Little birds are the members of a network of spies and informants in the service of Varys, the Master of Whisperers on the small council of the Red Keep. Many of Varys' "little birds" are seemingly innocuous street children.
Varys informs Catelyn Stark that he has "little birds" all over the continent, even in the North. He is already aware of the Valyrian steel dagger that was used in the attempt on Bran's life, and knew that Catelyn and Ser Rodrik Cassel were coming.
A young boy reaches Ser Jorah Mormont at the market at Vaes Dothrak, gives him a letter of pardon from King Robert Baratheon for his reports on the movements of Viserys and Daenerys Targaryen, and informs him he can go home. The letter clues Ser Jorah that an assassination attempt against Daenerys is imminent.
Ros continues to act as one of Varys' little birds, informing him of Littlefinger's plan of taking Sansa Stark with him to the Eyrie. Ros' arrangement is eventually discovered by Littlefinger, who then sells Ros to Joffrey for target practice before Varys can protect her.
In a meeting of the Small Council, Varys informs Tywin that Daenerys has come to power in Meereen and is growing stronger, with three dragons, two experienced knights at her side, and a powerful army behind her. Tywin decides to deal with her. He enquires of Varys whether his little birds can find their way into Meereen. Varys assures him that they can. 
Later, a young boy approaches Ser Barristan Selmy in Meereen and presents him with a letter. Barristan reads the letter and discovers that Ser Jorah Mormont was reporting Daenerys' activities to Varys.
In the books
In the A Song of Ice and Fire novels, at least some of Varys' "little birds" have had their tongues cut out (though Varys himself did not cut them out).