Powerful search-replace GUI app for Mac?

TextWrangler (free) or its big brother BBEdit (not free).

It seems that the version 1.0 of Find & Replace It! allows replacing in its trial version. Personnaly I like it for its preview feature and its scripting possibilities.

https://find-and-replace-it.com/

[update] New link

you don't have to pay a dime for anything! If your OS has bash/sed/find/etc and other *nix tools, you can do your own script to do find and replace! eg if you have sed

sed -i.bak 's/find/replace/g' *.txt