PowerGREP

'Am a database developer who frequently has to munge data having peculiar formats. PowerGREP is incredibly useful for these purposes. The user interface takes some getting used to. The vendor is consistently upgrading the product and has a generous policy for giving licensed users the right to use those updates. Their other product RegexBuddy is also top-notch. Highly recommended.

This thing is insanely powerful.
The interface is remarkably intuitive despite containing such an overload of features.

It is essentially used to regex through your filesystem.
It can search and replace strings inside any document or binary file.

It allows for creation of elaborate sequences of regex operations, which can be saved and reused as modules.

It could for example be used to find specific sentence structures in large volumes of documents, grab those sentences, compile them all together into a new file, and add HTML formatting to specific parts thereof.

Those who like regex or use it frequently really should try this wonderful piece of software.