Proraso Green - the eucalyptus and menthol one - becomes a fantastic lime-scented cream when you add just a tiny drop of lime essential oil to your shaving bowl before lathering. If you want to face lather, just bowl lather for enough swirls to get it mixed into your brush fibres. Seriously, it works a treat. Don't overdo it though - you need a tiny amount of oil, and too much may irritate your skin. Ylang ylang also works well.
I discovered this because I really dont like the scent of green Proraso, and happened to be rifling thorugh the oils when running a bath for my wife, when the idea struck.