is there an event in sharepoint that is raised when a user logs in?. I want to have a notification email whenever any user is logged in to a specific sharepoint site. Also, login here is not limited to opening the site from the browser, users might connect to the site using Microsoft Access or Excel. So, I want to track these cases as well
No there is no event for this. You have to craft something yourself (such as a HttpModule).
But first take a second thought if this is really necessary - what do you consider a log-in; every page load, every time a user open a document, only once per day etc etc