Do It is the easiest way for you keep on track. It stays in your menu bar for easy access. From the menu you can choose from your Presets (saved tasks you do regularly) or set a time and description for a new task. You can choose your custom alert, i.e. have a window pop up, window and an alarm sound, and/or have it open an application or file (i.e. a document you need to finish in when the timer goes off, or open an application like mail to send an email, or an Applescript). It's that simple!
Requirements: Compatible with Mac OS X 10.7 or later (Lion), 64-bit processor. Requires Mac App Store. Internet connection required for download only.