It's funny you should mention it, because just before I logged on here today I was thinking it would be so useful if this site had an app.
There is an app for the iPhone, it's called Abandoned. However... It only lets you post places in the US as it relies on a zip code. For US residents, it's great, you can upload pictures, and info, and there is mapping to, obviously if you don't want to give away the location, you just aim it towards the middle of the nearest town, or just somewhere else, but I often have a browse through it, they have some really interesting and MASSIVE places out there!
The Baker Hotel is my favourite!