Even if you don't know what Dexter is/all about, have a look at the trailer for Season 4 or just watch it right here. Pretty damn cool.
The Wikipedia article is full of spoilers but will tell you more than you ever wanted, let alone needed, to know.
Long story short, Dexter Morgan is a forensics analyst for the Miami police, and a serial killer by night. Trained by his father, a veteran cop in the same department (but now deceased), to kill anonymously without the cops being able to figure out who did it. His sister's also a cop, but doesn't know about his secret. Through his sister, he met a woman he's dating now, who was abused by her husband. Dexter only kills bad guys, people who have escaped justice somehow. He researches them, using illegal methods, to be 100% sure they are guilty before killing them. The show is seasonal, with each season being a complete story. Seasons 2 and 3 were sequels, not a continuation, of the first season. So it isn't necessary to watch all the seasons, but it does help with back story, and it's neater. Still, Seasons 1-3 were all good.
In other news, Jen and I watched the House Season 6 premiere, and that was really good. The ending could have been better (not to spoil anything, but I was hoping he'd spend the whole season institutionalized - whoops, I guess that was a bit of a spoiler) but the first part was very, very good. I'll have to watch it again, maybe I'll get more out of it. Personally, I'm waiting for another Doctor - more accurately, a Time Lord - the fifth season of the Doctor Who reboot should be starting early 2010, but we have two more specials - a Halloween special, and a Christmas special, before season 5 begins properly. And LOST will begin its sixth and final season soon. I'm just at the end of Season 3, so I hope taking a break to finish Potter 7 and the Dexter books Jen bought me for my birthday will allow me to watch the last three seasons at my leisure, and not have to wait a week for the new ones.