I've watched it twice now in the last month and it's one of those films that I cannot stop thinking about. I generally watch 4 or 5 films a week and since watching it I find myself just trying to find something similar but I've not managed to scratch that itch. I recently saw 'The Invitation' which has a similar feel but didn't quite hit the nail on the head.
It's the combination of Gyllenhaal's absolute madness but complete calmness and the dark, creepy undertones of the whole film. He really made that film great and I hadn't appreciated how good an actor he is until I saw it.
Any suggestions are welcome really, Ive gone through tastekid already but not found much.
A couple of similar films off the top of my head for reference would be; Blue Ruin, The Invitation (as above), The Gift, No Country for Old Men, Session 9. Running themes there I guess are a strange and unnerving character, dark visuals/scores, some violence and a lot of uncertainty. Thanks in advance, I look forward to watching some of your recommendations.
edit: Amazing suggestions you guys are great. I'm off to sleep now but keep 'em coming, I'll check all the new recommendations tomorrow! Thanks all.
edit2: Thanks for everyone's suggestions, didn't think I'd get such a big response but I've got a ton of new films to watch now. Time to find that one I've been looking for.