A picture of the icon might help a lot, but this can be quite a few things. Profile switching, IM app, etc..
Is it showing to the left or to the right of your notifications? If it's to the right, it's a system indicator (might be a Profile switcher or Contact manager). If it's to the left, it's a notification from an app you're running and most of the time you should be able to click on a notification to see where it's coming from.