"Todo list" or "to-do list"

Wiktionary defines both to-do and todo:

Noun
to-do (plural to-dos)
...
2. A task that has been noted as one that must be completed, especially on a list.
My to-do list has been growing longer every day.

And:

Noun
todo (plural todos)
(US) A task yet to be done; an item on a to-do list.

You can use whichever you want, but be consistent. To-do is a little clearer, but hyphens are naturally lost as languages evolve (to-day, wire-less), so todo should be fine too.

Edit: Waggers makes a very good point that todo isn't yet as widely adopted as to-do. So you may be better off sticking to to-do.

Ngram of "todo list" vs "to do list" (+ "list" because to avoid the fuss and commotion meanings, and "to do" will include "to-do"):

Comparison of term on newspaper websites, but note "to-do list" also returns hits for "to do list":

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

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.

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).