At the end of the episode "How I Met My Stepbrother", wherein Casey and Derek, having very nearly torpedoed George and Nora's relationship, rectify the situation with Casey proposing on behalf of her mother, and Derek accepting on behalf of his father.
There's the episode where Casey's father comes to visit and she tries to make it perfect, only for him to rush off after a short time. The only one who notices how upset Casey is is Derek, who secretly calls the father to come back. An Aww, Look! They Really Do Love Each Other moment.
Sheldon's proposal to Emily. It was really sweet and a great send off for the character.
There's an episode in the final season where Derek and Casey go to Toronto for a party, where Casey catches her cousin kissing Truman. Seeing Casey in tears causes Derek to suddenly become uncharacteristically caring and responsible. He chews both Truman and the cousin out for it, calls home to let Nora know what happened, and brings Casey back home right away.
Made even more heartwarming when Casey tells Nora that she thinks she was wrong about Derek.
Derek does this a lot. If Casey's legitimately hurt or upset by something (not just her overreacting), he'll drop the Jerkass act and do whatever it takes to make Casey feel better.