
Zavala makes a great simple task manager. You can break up tasks into smaller child tasks to complete. Once you have completed an item, you can mark it complete by using the row context menu or the ⌘-return keyboard shortcut. Zavala shows items are completed by dimming and striking through the text of that row.
If you have accumulated a number of completed items and want clear them you can use the Delete Completed Rows command from the Outline menu on the Mac or the More Options menu on iPhone and iPad. If you have a hardware keyboard, you can press ⌘-d.