Here's a case where a free app is better than a paid one.
There are dozens of to-list apps available for every operating system. I've tried many of them and even paid (too much) for the stunning Mac and iOS versions of Things. But I have since settled on the free, ultra-cross-platform product Wunderlist, and it's what I recommend whenever anyone asks me, "How do you keep track of stuff?"
What about Siri? No, thanks. Too iPhone-focused.
Wunderlist may not be able to listen to you, but it is simple. It lets you create multiple lists. I have Work, Personal, and Calls, for example, and I add trip-specific lists when I'm getting ready to travel. You easily pop items into and between them. What I like most about it, though, is how satisfying it is to mark an item as "done." You click on it, and it grays out and moves to the "done" zone at the bottom of your list, where it stays all day, reminding you of your victory.
What I really like about Wunderlist is that there are native apps for every device I use: an iPhone, an iPad, two Windows computers, and a MacBook. (There's also an Android app that I have installed on a Galaxy Tab, but that's not a productivity machine for me, so never mind.) And if that's not enough, there's a solid Web client. Wunderlist syncs all your items into the cloud, so no matter which of your devices you pick up, you'll always have your updated list of stuff to do.
Downsides? Wunderlist's Mac and Windows apps take up too much screen space; you can never make their windows as small as you like. Smart date processing ("Call Mom today") doesn't work on iOS. There are also no delegation features, which is actually fine with me; social features would add a whole layer of complexity.
Wunderlist is free, simple, and works on everything. It's my go-to for to-do.
That's not the end of today's recommendation. I use yet another list manager for grocery shopping: The free GroceryIQ app. It's like Wunderlist for food shopping, with a few special features that make it great: It sorts items by aisle (enter Cheerios and Syrup, and they'll both appear under "Breakfast and Cereal"), and it has a bar code scanner for entering items in your home you're running out of. It also has syncing, including to devices running a good universal iOS app with Web access.
Finally, my readers may ask, why not use Evernote for to-dos, since I apparently love that app so much I'd marry it, if it were legal? Because Evernote stinks for task management. I use it to remember notes and ideas, and I tried hacking to-do functionality into it for a while. Evernote is just not as fast, simple, or clear as Wunderlist for keeping track of stuff to do, or as GroceryIQ for keeping track of food to buy.
Platform: iPhone/iPod | iPad | Android | OS X | Windows | Linux | Web
Summary: A great to-do list manager that runs native on every platform that matters.
Platform: iOS Universal | Android | Web
Summary: Highly-specialized grocery shopping helper. Intuitive to use.