You can use the bibliography management tool JabRef (starting with version 2.10). Use the setting Preferences⇒File⇒Field saving options⇒Start field contents in same column. Just opening and saving the file will align the equals signs.
I’m not sure, however, what determines the number of spaces before the equals sign. Also, as you can see, JabRef sorts the fields according to what it deems obligatory and optional fields, and secondly alphabetically. I’m not entirely content with that functionality, but then again, if you’re using JabRef, you won’t be fiddling with the raw bib files too much anymore at any rate since you have the comfortable front-end for editing your bibliography file, as you remarked yourself in your answer about BibDesk.