There's a very cool script to add to your browser but first:
- Do you try to keep your inbox count in 0 (but usually fail miserably)?
- Do you like (need!) to be notified when a new mail arrives?
- Do you like Opera?
If you meet this conditions then you are very sick and need professional help. In the meantime, go and check Gmail Favicon Alerts 3, a cool script which changes gmails favicon to show your current email count. It works on Opera but it makes it crash. Most likely the script is not the one to blame here...