The Uppsala Alpine Center in Uppsala, Sweden is a good candidate
but I don't know if it is the lowest.
It is between the 0 and 5 m height lines on the map. I don't
have exact data but I would guess it's somewhere around 3, perhaps
4 meters above sea level.
There are also two resorts in Stockholm, Sweden, in
Saltsj�¶baden and Liding�¶ which are both situated by the sea
between the 0 and 10 meter height lines.
Kvisthamrabacken in Norrtälje, Sweden is 0,9 m above sea level
and is probably lower then the "resorts" mentioned above.
These are not really ski resorts but rather a hill with a lift
and no accomodation.