One episode of Angel (the one with the children's show puppets) had a kid named "Ratio Hornblower".
Monday Mornings had a nice Actor Allusion. Ioan Gruffudd played a doctor and his boss asked him several times if he was familiar with the concept of "captain of his ship".
Jake Sisko, son of The Captain in Star Trek: Deep Space Nine, is a writer. Whenever the camera peeks in on the manuscript he's editing, though, it's text from Commodore Hornblower—appropriate, as the franchise takes direct inspiration from Hornblower.
In one Honor Harrington novel, the title character is reading a Hornblower book. And, in fact, the whole series had its genesis as a "Hornblower In Space" book; the matching initials are a deliberate Shout-Out and the first book is dedicated to C.S. Forester himself.
One book in the Alan Lewrie series (depicting a much less heroic naval hero) has the main character notice of a tall, thin, and melancholy-looking lieutenant in a threadbare uniform at the Admiralty during the Peace of Amiens—a tacit reference to Hornblower, who was without either a ship or half-pay during that time.
The third Ciaphas Cain book mentions a navy commander named "Horatio Bugler."