Jeeves and Bertie Wooster. Bertie describes himself as a 'Nature's bachelor', and seems to get very upset when he has a tiff with Jeeves. There are also the 'tender glances' and comparisions between Jeeves and the wives and sweethearts of Bertie's friends. Jeeves, on the other hand, practically goes ballistic (by his own inhibited standards) if Bertie ever gets engaged and goes to great lengths to keep his master unmarried. Of course, he is stated to have a strict policy of never working for married men. It's up to the reader which way to take this, as it's probably unintentional. (To some people, Bertie gets entirely too upset for an employer when Jeeves gives him the cold-shoulder.)
In the very first episode, Aunt Agatha describes Bertie's perfect wife - a description that fits Jeeves like the most fashionable of gloves.
Jeeves certainly seems pretty upset when Bertie wears clothing he dislikes. There's an instance in one of the stories where Bertie makes up with Jeeves after some disagreement over clothing, and then he says that he feels like a hero in a story who made up with his wife after a quarrell.
There are those that have considered "Thank You, Jeeves" a break-up, make-up story when Jeeves leaves Bertie's employ due to Bertie's new habit of playing a banjolele. After much chaos involving black-face, Bertie's new valet being a thief and setting fire to Bertie's cottage (which destroys the banjolele), and engagement confusion, they get back together.
The two greatest Ho Yay Jeeves stories are considered to be in "The Aunt and the Sluggard" and "Bertie Changes His Mind" (both from the book "Carry On, Jeeves"). In the former, Bertie is forced out of his own apartment and has to stay alone at a hotel, prompting several pages of him angsting about how miserable his is without Jeeves. The latter is the only story in the canon narrated by Jeeves. Many a troper's suspicions were confirmed when Jeeves refers to his and Bertie's situation as "our cozy bachelor establishment."
Personally, this troper finds the most Ho Yay instance in the Jeeves & Wooster stories to be the American end of 'Jeeves and the Tie That Binds', where Jeeves destroys all the pages in the Junior Ganymede's book he's written about Bertie because he intends to stay with Bertie 'indefinitely', and when Bertie asks what keeps Jeeves with him despite all their arguments, Jeeves tells him there is a 'Tie That Binds' that keeps them together. Also, there is a scene at the end of 'The Inimitable Jeeves' where Bertie is fully prepared to fire Jeeves for what he's done, but he comes in and sees his perfect apartment and perfect valet and is utterly pacified.
Note closely above the name of Jeeves's club in London, for valets ... the Junior ... (ahem) ... Ganymede Club.
Psmith and Mike - In just about every book, Psmith will make a reference to them going off 'hand-in-hand' together. He 'hires' Mike as his secretary so as to share rooms with him and later convinces his father to sponsor Mike through university so as not to be split up from him.
Not to mention the beginning of "Psmith, Journalist," where Psmith spends a good deal of time moping about the fact that Mike has to go off and play cricket.