When Simon types out 'save him' on Celty's PDA when she asked about Kida.
You might have to squint a bit to see it, but in episode 23, when one of Saika's children stops Horada from killing someone, because "it would make Mother sad." He wasn't ordered to intervene, he just did, proving that despite all of Saika's Blue and Orange Morality, her daughters really do love her.
Episode 24, when Masaomi and Saki reconcile.
For a light novel example hereand here. Warning, there probably are spoilers. Read at your own risk.
On that note, as creepy as their relationship is, there's something adorable about the way Seiji and scar-neck girl/ Mika are holding each other in that ED.
In the newest light novel, there is an example after Shinra gets stabbed where Izaya says, in a very tiny voice, "... can you tell them I stabbed you?" I don't know, seeing Izaya like that is kind of a heartwarming moment for this troper.
Same here! This just shows he's not a complete asshole that everyone think he is.
For this troper, after reading the entirety of volume nine any scene between those two becomes an automatic Crowning Moment of Heartwarming. This troper's someone who manages to relate to Izaya and has her own Shinra, so their interactions really hit home for her.
In volume nine's epilogue where the Orihara twins admit that as much as Izaya treats them like anyone else, they'll still think of him as family.
In the Everyone Gets Along omake, Mikado reveals that he cried when he found out that Kida was moving to Tokyo, but is still too embarrassed to tell him. It's not really surprising considering their relationship, but you can't help but gush about it anyway.
Kadota's band reuniting with Kaztano in episode 6, after working all night to rescue him from kidnappers. Followed by the six of them going for breakfast at Russia sushi, happy as can be.
In episode 25 we have a small, subtle example but an example nonetheless. As a man with a knife tries to kill Kasuka he's suddenly hit by a sign, obviously thrown by Shizuo. Kasuka turns to look at Shizuo hiding in the crowd. And when Shizuo splits, Kasuka gives a tiny smile. Perhaps the only genuine smile the guy has had on the show. And it's adorable.