Both Ack and The Silver Searcher have a -g PATTERN command that enables for fast file listing based on expression matching the file path itself, while still keeping in mind configuration settings (like ignoring certain file types from showing in results).

While this -g flag enables these applications to perform quite differently (matching against path/filename rather than actual contents), it allows them to be used in interesting contexts, such as the driver to CtrlP indexing. More info regarding the benefits of this use case are spelled out in @thoughtbot's blog post "Faster Grepping in Vim".