There is sometimes a PM or email from someone claiming to be a friend or fan of the event/celebrity asking for pictures of an event that you took for the fun of it and that they will give it to the celebrity/event organiser. Just let you know its usually the event manager or even the photographer who missed the shot that you got. If they say is official fan club, then check it out as usually will have website/facebook.
Now then depends. Are you convinced your photos are good enough that you intend to charge for it or just give them the photos with credits in the exif and image itself because not up to "commercial" standard?
Just remember, if you give free most likely they will expect free next time but giving free once usually means your email/cs nick will be entered into a database in case they have event going on and after that they will search CS for images of their event and ask for the images. The thing about doing it this way is that even if you charge them for the images, chances are you will still be in their database because seriously, they wont remove your entry unless for some reason they are really pissed about something.
Then there are times you get invites or read a post about a product launch that needs "volunteer" photographers. Usually they have their own photographer already just that it would look nice to their stakeholders if suddenly so many photographers with "specialized" gear were to appear to give illusion of media presence. Its either an open event or by invite as in they want to see portfolio or know what gears you have and intend to bring. Bigger is better often for these launch.