I'm happy that the 'basic weapons' have one modification slot, but what weapons have 2+ slots?

Legendaries, god weapons and the Swashbuckle. Possibly some others.

EDIT: Also The Ether

Where can i find a list of them?

I don't know that you can find a list of weapons by their number of mod slots. Check the battle forum? You can find a list of Legendaries here. Also, I edited my post above to include the ether.

Maybe you've already ran across it, but you could try the List of Modifier Crystals thread. They have a nice little section of weapons with their number of sockets.

Thanks :D