One of my few hobbies is urban exploration. This normally means just looking around a site, normally an empty or disused building or factory, and being nosey and taking photos.
Many are easy, but some require a certain level of stealth and cunning :)
For those that are worried, various safety precautions are carried out, and there is no crime being committed and so no chance of being arrested.
There are quite a few sites in the urbex community, so here's a selection:
Silent UK - SilentUK is a collection of stories and photographs from various adventures across the world all which fall under the term, Urban Exploration.
World Abandoned - World Abandoned Travel to strange and scary places with World Abandoned! The blog features videos, pictures, stories and facts from ghost towns, old attractions and sites lost to history.