PopChar has been around for over 20 years and is a favorite of editors and designers. To type a special character, you click on a P in the menu bar and a list of characters appears. Selecting the character you want drops it into your current document. Sure, you could always use Apple's Character Palette tool, but it's slow, and difficult to search for a special character in a particular font.
PopChar X 4.0 adds a new feature for searching Unicode characters by name across font boundaries. Ergonis, developer of PopChar, provides an example of searching for a "cubic meters" symbol in Helvetica. Typing in "cub" produces no results, but you can click a new "All" button to search across all Unicode fonts.
You can download a trial version of PopChar X 4.0, or purchase it online from Ergonis for €29.99. Multiple license packs are available at a discount.