The Software Behind
There are a lot of good ones out there at the moment, so without further ado, here is your list
The two that come to mind in this category are Things and OmniFocus. Both have iPhone and accompanying Mac apps and both sync seamlessly between the two platforms. Things lacked this functionality in the past but, it has recently been updated. Full disclosure, I am a super-fan of OmniFocus and I also feel anything designed and or nudged by Merlin Mann and Ethan Schoonover is worth checking out / using until they make something new.
Regardless of bias, both of these applications will work well for your daily to-do's, with OmniFocus being much more tailored to the GTD methodology.
The two that we will talk about (there are quite a few in this category) are the Apple Reminders app and Wunderlist. Both of these apps will set you back a grand total of 0 Yankee Dollar-y-Doos and both will sync between your Apple powered devices. Wunderlist will even work in a browser, Windows, and Linux. Both of these are more traditional list managers and have fewer (read: less complex) frivolous features.
If you are in the market for a sleek, simple, and free app, take a stroll and give one of these a whirl.
The three we will go over briefly (because really to get an idea of how they work, you should be trying them out) are Asana, Chedder, and Flow. These applications aren't only for the web, all three of them have a mobile component, and Chedder even has an early stage Mac client. Chedder is a really simple and fast set of lists where Flow attempts to recreate the Things experience (half-hearted GTD) on the web and Asana is great for working with teams.
There is a really quick, and dirty rundown of the current crop of task managers. My recommendations? Give each one a whirl and see what fits your lifestyle. Some of us have complex, "big boy" needs, while others benefit from a simple list manager.