Triggered by recommendations from Will Newsome and Gwern I watched all episodes of Death Note in like 3 days and it's by far the best series I know of. I only comment here 'cause I feel really sorry for all the folks out there who haven't watched Death Note yet.

Here's a short overview by Gwern (spoiler warning!):

The anime/manga Death Note is interesting on many levels. The shinigami are unexpectedly Japanese versions of the Grim Reaper; the cosmology is remarkably atheistic1; its genre is almost unclassifiable (detective/mystery, drama, suspense, action, Greek tragedy, supernatural, or what?); the plot warrants considerable analysis, and furnishes considerable scope for thought - just coming up with ingenious ways of exploiting a Death Note consistent with the rules is good intellectual fun, or one can devise interesting theories to analyze (example: could one use information theory to devise optimal strategies for a Kira and a L?).

And here the beginning of a short plot summary (again by Gwern):

The plot is the important thing. The brilliant, ambitious & altruistic Light Yagami comes into possession of a small black notebook called a ‘Death Note’, which allows him to kill anyone he wishes by writing down their name.5 (The Death Note was abandoned by a supernatural being, the shinigami Ryuk, who watches everything Light does with his old Death Note and occasionally helps Light in exchange for delicious food.)

Naturally, Lights begins to use the Note and starts by systematically killing off criminals listed in public records. Arrogantly (or hubristically?) he makes no attempt to conceal his murders, and eventually police all over the world begin to notice that an untimely number of criminals are dying of heart attacks. They dispatch their best detective, L, to hunt Light down and bring him to justice. (L is not his real name, but L, N, & M all adopt pseudonyms to foil the Death Note.)

By clever ploys, L manages to narrow down Light’s location to his town, and begins hunting there in person. Light turns the Japanese police against L, but at the cost of narrowing down the pool of suspects so drastically that L puts Light at the top of the suspect list.