Because Airliners have a tendency to change operators quite often it's tricky to keep tabs on who's flying a particular aircraft at any one time. !!!!!!!!!!!!!! is the favourite site for internet plane watching because there are loads of contributors, most of whom are busting to get photos published. Despite having a high proportion of geeks (the forums have to be seen to be believed!!) it's a good resource, simply thanks to the sheer amont of images available, being added to even as we speak.
Myself I take 'Aviation Letter' every month, this lists all the latest changes of Owner/Operator/Tail#, but it's paper-only and can be a week or two behind what you'll actually see at an airport.