Video games, mostly. Some games draw me in more than others.
Minecraft for example. To be fair though, that one's not exactly an abnormal obsession; the game keeps players interest by offering about 10 kazillion things to do and ways to play it. Even so, my interest in it has never faltered since the game first appeared.
Games that arent as well known can do it too. Dwarf Fortress, for instance. And I'm playing alot of AI War lately (found on Steam, it's basically the Dwarf Fortress of RTS games). And Nuclear Throne.
I'll play those games absolutely to death, with other games switching into their places every now and then.
I also tend to be very obsessive about characters, either from games or from anime/manga. I have the resources to go to anime/gaming conventions whenever I happen to feel like it, and I'm into cosplay, so that obsession manifests itself in that form. Sort of a chance to BE the character for awhile. And further stuff, such as figures, images that are typically used as backgrounds for Windows or my iPad.... and so on.