Custom TODO mark in Eclipse

I did it like this on my Mac Machine,

Eclipse>Preferences>Java>Compiler>Task Tags >New

Now eneter the TAG and set it's Priority (viz. Normal, Low or High).

Apply>OK

A dialog will popup and will ask you for a complete rebuild, tell it to rebuild now. You are good to go after a rebuild.

Window > Preferences > Java > Compiler > Task Tags

You can add new tags to your heart's content.

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NOTE: Why the Eclipse guys made this a Compiler setting is beyond me. It just makes the setting hard for people to find, so it rarely gets customized.

The answers by benjismith and j pimmel are perfectly valid and correct. I just want to add that you can find this and other settings by using eclipse's neat preference-dialog feature:

When you open the preferences dialog via "Window/Preferences", type "todo" in the text field in the upper left corner. The options tree will be limited to matching settings.

Voilá