Tsubasa: RESERVoir CHRoNiCLE (TV)

This anime features various characters from CLAMP's manga works, such as Sakura from Cardcaptor Sakura.

The magic circle used by Yukito to teleport Syaoran and Sakura is a variation of the magic circles seen in Card Captor Sakura, except that all the celestial symbols are lunar. The CCS circles usually had a sun, star, and moon with one of the three as the "principal" element.

The little fluffy sidekick called Mokona is identical to the one in Magic Knight Rayearth, except in Tsubasa Chronicles it can actually speak.

In addition to other CLAMP characters in the cast of Tsubasa Chronicle, Watanuki, Yuuko, and the black Mokona (which stays with Yuuko and Watanuki instead of travelling with Syaoran, Fye/Fai, Kurogane, and Sakura) are from xxxHOLiC.

Syaoran, Kurogane, and Fye's top-rank Kudan forms are taken from the Mashin in Magic Knight Rayearth: Rayeath (the wolf representing fire), Celece (the dragon representing water), and Windam (the pheonix which represents wind).

The hairpin Sakura wore in ep33 (Ashura's history) is similar to the one she wore in the cardcaptor movie after Yukito bought it for her

Miyuki from "Miyuki-chan in Wonderland" appears in almost every major world Syaoran, Fye, Kurogane and Sakura go to. She offten appears with a piece of toast or another bread product in her mouth

Oruha (aka the Lounge Singer or I-1 in the Outo/Edonis World arc) is originally from the manga series Clover, where she was the 'One-Leaf' Clover. In the Piffle World arc, her Dragonfly vehicle is also named Clover.

The Swords Kurogane and Syaoran get in the Land of Outo, Souhi and Hien, bear the same names as Princess Hinoto's two servants in X manga, also made by Clamp

In the land of Outo, man named Seishiro is looking for someone named Subaru. They both, Seishiro Sakurazuka and Subaru Sumeragi, appear in two other series: first in Tokyo Babylon, then in X/1999.

At least four characters from another CLAMP series, Chobits also made appearances in episode 1, 22 and 24. In episode 1, persocom Chii talked to Fye right before he went to see the Dimension Witch. Persocoms Sumomo and Kotoko were the super-sized onis on the top floor of the tower in episode 22. A similar person from Chobits named Chitose was featured in episode 24.

Shougo (first appears in episode 2) is from X movie, where he was also a water element user. His girlfriend, Primera, is a tiny fairy from Magic Knight Rayearth.

In episode 3, 2 characters appear who help Syaoran and the others, that are form CLAMP’s work, X-TV, Arashi Kishuu and Sorata Arisugawa, and like in the original anime, they form a couple.

In episode 11, around 18:50, the mitteishu reveal themselves. The characters are a reference tho the series "CLAMP School Detectives".

In episode 13 the legend that is the center of the story features a blond haired Princess named Emeraude, a reference to Rayearth's Princess Emeraude.

In episode 14, around 12:58, a child is holding a stuffed animal that looks like a black colored version of Chiyo-chichi (Chiyo's so called father) from Azumanga Daioh.

In episode 18, we get introduced to some crossover characters. You have Yuzuriha Nekoi with Inuki and Kusanagi Shiyu from X/1999. They also appear briefly at the end of episode 17. Aftwerwards, the information, monger Erii is a character from the "Duklyon: CLAMP School Defenders" manga

In episode 19, the bartender that appears is Caldina from Magic Knight Rayaerth

In episode 21, we get introduced to Seishiro who is another CLAMP character from Tokyo Babylon and X/1999

Chi can also be found in episode 32. When Chi and Fai goes "out"

In episode 32, the the cafe Fay takes Chi to is called Tirol - which is the same name as the bakery Chi works at in Chobits.

In episode 44, a deity called Cerberus appears. He is also a protagonist of the Cardcaptor Sakura anime.