xxxHolic currently encompasses two anime series, a two episode OAV, and a movie release. This particular review will cover the original series which is currently available through FUNimation in the states. It is simply titled xxxHolic, and is typically pronounced "Holic."
xxxHolic centers on Kimihiro Watanuki, a high school student who can see spirits, and is also a magnet for spirits. His life is impacted by fellow classmates Shizuka Doumeki, whose family runs a local shrine, and Himawari Kunogi, whom Watanuki has a serious crush on. They have all been connected by a mysterious woman calling herself Yuuko Ichihara, who runs a shop that grants wishes. Keeping Yuuko company are the cheerful girls Moro and Maru, and a black Mokona (just like white Mokona from Tsubasa).
One day Watanuki is being chased by oppressive evil spirits, and while trying to escape from them he passes through a fence that surrounds Yuuko's shop. Yuuko offers to grant his wish (what that is isn't clear, but one would assume it has something to do with his ability to attract and see spirits), but she never does anything for free. In exchange Watanuki must work in Yuuko's shop. Mostly he cooks for her (his father was a chef, and he picked up the skill), but he also cleans and runs errands. He also manages to lend a (typically clumsy) hand in Yuuko's wish granting.
According to Yuuko, everything is inevitable and there are no coincidences. Everyone who comes to her shop arrives for a reason, and if they have no reason they will never even notice the shop exists. Included in this inevitability are the meeting of Watanuki, Himawari and Doumeki. Along with Watanuki's powers, these two classmates have powers of their own. They're not made explicitly clear within this series, but it seems that where Watanuki attracts spirits, Doumeki repels them (though he cannot see them himself). Himawari appears to have the (unknown to her and...well probably everybody but Yuuko, though she doesn't say anything) ability to break seals (ah, a quick wiki search says that she causes misfortune to those around her, which would include breaking seals holding back evil powers, I suppose).
The show goes episode by episode, typically with a different customer with a different problem arriving in each for Yuuko to help, or Watanuki to attempt to aid in his own way. There are a few recurring things that tie them together, like the Zashiki-warashi (a sprite) that has a crush on Watanuki, and the fox spirits who own a food stand. Yuuko's customers have various problems like habitually lieing, an addiction to computers, and a woman who always makes irrational decisions. Scattered between these customers with "bad habits" are episodes involving the entire gang telling ghost stories, spending the night in haunted houses, exploring superstitions, and learning about their powers.
Fans of Tsubasa Chronicle will recognize several of these characters. The Tsubasa gang (in their respective show) arrives at Yuuko's shop to gain the power of moving between dimensions (the white Mokona). This isn't really touched upon in the xxxHolic anime at all (you never even see white Mokona, though Yuuko does mention there is a second); it's never mentioned, though Fay's staff is seen in an episode. This seems to be sadly due to a licensing issue with the anime studios. This connection is, of course, in the manga. There are a few other Clamp references in the show (like a pair of Chobit ear pods Watanuki uses as headphones).
The animation style is pretty unique. The characters have extremely (absurdly) long limbs, and there's an angular almost blocky style to their designs. The intro song is excellent, though there isn't anything really noticeable about the score other than it fits the show (which is the important thing anyway).
xxxHolic is a 24 episode series, currently available on 6 DVDs and in box sets. Watched on the Funimation channel with a good English dub, except for Watanuki, who is almost intolerably voiced by Todd Haberkorn. Though if we want to be honest here, Watanki is a pretty annoying character; the show needs more Doumeki.
Pictured: Watanuki, Yuuko, Doumeki