I've had a Hive door sensor for a while now, as regular readers will recall. Recently I wanted an similar device for a side project at work. In that case, there's no pre-existing Hive infrastructure, so the Hive option didn't make financial sense.
Energenie do a nice door sensor for £20, but as with all these devices, it speaks to a hub which needs buying separately. I managed to find one of theirs online for £40, which is a bit much for such a simple thing, but the cheapest option out there.
The Energenie app isn't as nice as Hive (the notifications say "a sensor has been opened" but you have to tap to see which one, which is a bit silly). It does however talk to IFTTT perfectly.
I suspect it can also be made to talk via their Raspberry Pi connector, but I haven't had a chance to try that yet - and I must admit IFTTT + web hook was really easy to get working without writing much code.