Assuming you’ve done the same thing then you know the dangers of a recommendation. My basic philosophy is to recommend only those Mac apps I’d use myself or recommend to family; knowing the basic customer support obligations that come with it.
For Mac users who don’t want to spend money, but want more than a todo list app, and find Reminders and Notes in OS X to be somewhat anemic, I can offer Wunderlist, a free Mac task management app that works well with David Allen’s Getting Things Done philosophy. The whole idea behind GTD is focus and discipline and Wunderlist makes it easier to manage both.
What you get is a simple method to create lists, share lists, attach notes, photos, or PDFs to a list, and manage reminders for items on your lists. The user interface is almost– almost– self explanatory. Categories and lists in the lefthand sidebar, click to get specific details, click again for more details on each item.
Setting reminders for specific todo list items is almost as easy as Reminders in OS X, but does more. Folder organization means you can create and maintain many lists and treat them as tasks within a project (folder). Wunderlist is more powerful than any todo list or reminders app.
Usually, when I recommend a Mac app I check to see which smartphone or tablet a friend or neighbor is using to make sure there’s a companion mobile app. Wunderlist has an app for almost every platform; Windows, Chrome, Android, Windows Phone (or, whatever they’re calling it these days), even Kindle.
Free? Did I mention free? Well, kinda sorta free. It’s free to use but the Pro version, good for businesses and team collaboration, comes with a price tag. Otherwise, Wunderlist is hard to beat for the price.