For instance: if you have a view that produces a list of titles of nodes of content type X, how can you make that list exclude the node that is currently displayed to a site visitor?
This can be done with a regular argument.
Create an argument for
Node -> 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.