Are there any sites similar to netflix?

In terms of subscription-based, Sky Go is the closest equivalent, although much more expensive at €18. It gives you access to all their On-Demand content and 20 Live TV channels. They've a good selection of HBO shows via their Sky Atlantic channel, but apart from that their selection is pretty average. There isn't much there in terms of Sitcoms.

The other alternative is Blinkbox. They've a great selection of TV Shows to purchase, but you'll have to pay per episode/series.

You could also try setting up an US iTunes account. They've a fantastic catalogue, but again is pay per episode/series. But you do get many options, with HD episodes available and all playable on iPhone/iPad/iPod/Apple TV along with your PC/Mac.