On the short list of my all-time favorite TV shows (along with Buffy, Firefly, Veronica Mars, The Wire, and a few others) is an NBC show that ran for seven seasons (two of them unusually short), called Homicide: Life on the Street. (After it was canceled, a two-hour TV movie wrapped up the remaining plot points.)
Like the still-ongoing (and equally if not even more amazing) HBO show The Wire, Homicide arose out of the work of journalist David Simon. Simon was a Baltimore reporter who took a year off from his newspaper work to follow around the homicide unit of the Baltimore Police Department. Out of that experience came a nonfiction book, called Homicide: A Year on the Killing Streets. The book was turned into a fictional TV show, with characters based on their real-life counterparts, and plots (and even dialogue) taken directly from the book. Simon was not initially involved with the show, although he did get involved in the later seasons.
The Wire was created, executive produced, and so forth by Simon (along with his collaborator, ex-PBD cop Ed Burns. One way to look at The Wire is Simon's attempt to rewrite or rework what he didn't like about the TV version of Homicide; see, for example, this amazing blog essay by Marc Singer about how various Wire characters are rewrites of Homicide's Frank Pembleton.
Homicide is different from The Wire: for the most part the episodes are self-contained (unlike The Wire, where each season is a single film fairly arbitrarily divided for broadcast purposes); the show is just about the police, rather than also having the investigated people as co-equal characters; it's not an HBO show, and that effects a lot of elements; and it's not quite as grittily realistic in its refusal of narrative conventions as The Wire is, especially in its later seasons. Obviously Simon had his critiques of Homicide, notwithstanding his own involvement both as the author of the source material and, later, as a writer and executive producer on the show. But saying Homicide might not be quite as good as The Wire is like saying that Bush might not be quite as evil as Cheney: given the magnitudes involved, such distinctions ultimately don't mean much. (And some people like it better: some people reasonably like that you don't need to watch an entire 12 or 13-hour film to get the aesthetic pay-off, for instance.)
Despite all that, Homicide is some amazing television, unlike any other cop show (except, naturally, The Wire): gritty, realistic, fabulously dramatic and powerfully bleak, it's just superb. It is well-directed and well-acted -- Andre Braugher (playing Frank Pembleton) is widely (and correctly) seen as the show's anchor, but most if not all of the other actors are also astonishingly good. (Although Braugher's absence in season seven is one reason that Homicide's last season was its weakest (he returned for the wrap-up TV movie.)) It is, quite simply, great TV. If you don't know it, give it a try; if you like The Wire and don't know Homicide, you should definitely check it out; if you liked Homicide at the time, but don't remember it... read on.
Right now -- and I haven't a clue how long it will last -- Amazon has an amazing deal on something called the Homicide Life on the Street Complete Series Megaset. The set contains all seven seasons, the TV film, the three cross-over episodes that Homicide did with the vastly inferior (but vastly more popular) show Law & Order, which aren't included in the usual single-season sets; and a variety of documentaries and so forth. It's basically the entire show with all the trimmings -- for $97.49. Given that individual season sets run from $45 - $85, with the Amazon discount, this is really an incredible deal. Even if you have a few of the season sets already, this is still the cheapest way to get the others; and if you don't -- well, it's clearly the best way to get one of the best shows ever to air on American TV.
So if you want to get the show on DVD at any point, now is the time to get it. And if you haven't wanted to in the past, but like watching great TV on DVD, consider getting it. It's really worth watching -- and rewatching.