What argument to use in Views that excludes the node currently being displayed?
This can be done with a regular argument.

Create an argument for Node -&gt; ID, choose Node as the validator and Node ID as the argument type. Then just tick the Exclude the argument check box. You can also choose specific node types here if you want.

Used with a block, this will list all nodes that aren't the one currently being viewed. You can then filter it further to suit your needs.