"Todo list" or "to-do list"

While "todo" is found in some dictionaries (such as Wiktionary) it is less common in more established dictionaries such as the Collins English Dictionary, and the entry in the Chambers 21st Century Dictionary only lists the hyphenated form. While Hugo is correct that often the language evolves and hyphens are eventually dropped, this process takes time and at present todo is not widely recognised as a single word in the way that "today" and "wireless" are. Indeed, this Google Books N-gram illustrates this very clearly:

I would therefore advise against using "todo" and side with the spell-checkers.

The OED has to do and to-do, but not todo. Todo looks like an extinct bird to me.

In short, both are correct. Though in the purest sense, it could be that neither are.

Most dictionaries seem to contain only the classical definition ("a disturbance or fuss") rather than the modern one that most people use today ("something to be completed"). I was unable to find any "task" definition other than in Wiktionary and The Free Dictionary; at least online, it's missing from the New Oxford American Dictionary, Collins English Dictionary, Chambers 21st Century Dictionary, Merriam-Webster, Cambridge Online Dictionary, and Longman Dictionary of Contemporary English. Can anyone confirm this?

According to the Online Etymology Dictionary, To-do came into use in the 1570's, meaning "proper or necessary to be done". A newer meaning emerged in the 1820's, being "a disturbance or fuss". I saw someone quote a 1920's figure for a resurgence of the "task" meaning, but according to the N-Grams it looks like that didn't happen until at least the 80's.

The few dictionaries that recognize the "task" meaning generally consider both to-do and todo as proper. Though as others have noted here, to-do is vastly preferred by formal publications; only in the programming world and some informal contexts does todo seem to be more common.

TODO (often in all-caps like that) is often used by programmers as a comment in their code (or in their presentations) to denote that something still needs to be done.

From Jalayn's answer on the software engineering SE site:

Modern IDEs recognize the TODO comments and they are as such visible in their own panel/window/tab, so they are theoretically not lost (I'm thinking Eclipse and Visual Studio, both I know enough to remember that they recognize it).

I've also been told by computer scientists that it's not pronounced like to do, but it's pronounced as a single word tɵdɵ.