I recently learnt that plus addressing is supported in GMail. I feel this is a nice feature. If plus addressing is supported, even if we send a mail to email@example.com, it will be delivered to firstname.lastname@example.org. But, for user, it will be easy to set filters based on the extra information (+office). User can give different email addresses (user+office, user+maths, user+studies etc.) to others for different purposes.
This feature (but as minus addressing) is supported in Yahoo mail also (only in Yahoo Mail Plus). You can read more information about plus addressing here.