After First Monday was cancelled, Sheffield became a recurring character on JAG.
NCIS backdoor pilots are JAG 8x20 and 8x21.
NCIS LA pilots are NCIS 6x22 and 6x23.
The NCIS episode "Endgame" (7x07) continued on the events that had taken place in the NCIS: Los Angeles episode "Killshot" (1x05).
Abby guests on NCIS LA 1x09
Kensi from NCIS: Los Angeles also guests in Hawaii Five-O 2x06, they mention this in NCIS LA 3x06
A.J. Chegwidden is making guest appearance in NCIS 10x24
NCIS: LA and Hawaii Five-O cossover: HFO 2x21, NLA 3x21
NCIS: New Orleans pilots are NCIS 11x18 and 11x19
Hetty also guests in Scorpion 1x06
Gibbs helps out Pride in NCIS New Orleans 1x03
CGIS Agent Abigail Borin known from NCIS seasons pops up in NCIS NO 1x12
NCIS: Red pilots were NCIS LA: 4x18 and 4x19 but the concept was not taken up.
DiNozzo asks help from the LA team in NCIS LA 7x05
Sister City: Part 1 is the 12th episode of NCIS Season 13 and the second part, "Sister City, Part II" as part of NCIS: New Orleans Season 2.
The story plot loosely connected between MacGyver 1x15 and Hawaii Five-0 7x17
In MacGyver episode “Flashlight” (1x18), Mac and the team team up with Chin and Kono from the Five-0 task force. They mention this in Hawaii Five-0 7x19.
NCIS 14x15 and NCIS New Orleans 03x14 are a two part crossover, Pandora's Box .
A.J. Chegwidden pops up again several times in NCIS LA Season 8 (8x15, 8x21, 8x22)
Bud Robert Jr. from JAG guests in NCIS 14x1
Noelani Cunha from Hawaii Five-0 appears in Magnum PI (2018 ) pilot and 1x14 episodes
Kamekona from HF0 appears on Magnum PI several times (1x02, 1x13, 2x06, 2x12, 2x13, 2x14, 2x18)
Gibbs visits NCIS: New Orleans 5x01
Lead roles of JAG: Harmon Rabb and Sarah MacKenzie show up in NCIS LA 10x23 and 10x24
HFO 10x12 and Magnum 02x12 are a two part crossover
HFO 10x18: Rick from Magnum PI helps Five-0 with a case
MacGarret was mentioned in NCISLA 11x11
Sarah MacKenzie show up a couple of times in first and last episodes of NCIS LA Season 11
Adam Noshimuri pops up and Tani is mentioned in Magnum 2x20
NCIS s19e17 / NCISH s01e18 - two parts crossover
NCIS s19e19 - character appearance form NCIS Hawaii
NCIS s20e01 / NCISH s02e01 - two parts crossover
NCIS s20e10 / NCISH s02e10 / NCISLA s14e10 - three parts crossover