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Wunderlist is great, available on all devices and all platforms. It has a free and a paid version and is interestingly good when tasks start getting out of hand and a good organization is needed. Wunderlist gives both folders and tags to organize alongwith some context oriented features and smart lists.
I use a webapp, Checkvist due to it's ease of use, ability to nest checklists/tasks, insert (and hide) notes, and flexibility (it also excels as an outlining tool). It's one shortfall in regard to your checkbox requirement is that it doesn't have an "active" checkbox. You can view and print the checklist with a checkbox; however, to indicate competed items ...
I think cherrytree might be what you are looking for. http://www.giuspen.com/cherrytree/ You can insert check-boxes (Edit->Insert special character) and tick them or untick them with a click. http://www.giuspen.com/topic/cherrytree-on-mac-osx/ The image is from Linux, but it's also available for Windows. I don't see it for OSX, but somebody got it ...
I have been through Evernote, Stickers, Remember the Milk and I didn't like any of them. I use my computer AND my phone to organise my things. Evernote cannot store as many notes as you might think. At some point it pops up and requests money. I cannot connect Stickers to my phone. Remember the Milk can only synchronise once every 12 hours or so. I use ...
I've personally used Stickies for years and I think that, for a "Post-It" like application for Windows with the ability to set reminders, you'll be hard pressed to find better. Doesn't synchronize with any online service, though.
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