https://play.google.com/store/apps/details?id=com.brapeba.roaminginfo
Shows the logo of your current mobile carrier in the status bar.
Displays a notification with current operator's name and in case of roaming it also shows the original SIM's operator name.
If you swipe (dismiss) the notification, a new one will be displayed when a change in the conexion occurs. To remove the service from memory and avoid further notifications, just tap the notification.
Double click the app icon to show the settings activity: to enable/disable sound, vibration and data notifications.