As ADDers, our built in internal clocks don't work so well out of the box. So we need to use external clocks as reminders, until/if we develop a better internal sense of time. This is not meant to be an exaustive list of every type of software, rather it's designed to show some possible categories and examples.

Online Based generally cross platform, i.e., (Mac/Win)

Online SMS (text messaging reminders)
Online Email Reminder Services
Online Day Planners regular and GTD type
Online Email Add Ons, Firefox Gmail GTD Plugin
Online Task Reminders, Remember the Milk, Backpack, Ta-da List
Goal Tracking Software, Joe's Goals
Web Based Project Management Software. Home Projects or Business ones, Basecamp
Wikis, GTD TiddlyWiki, (a reusable non-linear personal web notebook) MonkeyGTD, D3

Computer Based

Daytimers
Reminders
Alarms Stopwatches, I.e., I'll only stay online for 10 minutes:) Pester, (Mac)
Email Add on programs, i.e., Mail Act On (Mac) GTD Outlook Plugin (Win)
Outliner/Mindmap Software for wrangling your divergent ideas into a convergent plan, OmniOutliner (Mac) ConceptDraw Mindmap (Mac/Win)
Project Management Software, Omniplan (Mac) ConceptDraw Project2
(Mac/Win)
Wake Up Devices for Internet Addicts, Timeout, Flextime,

External Devices

Timers The best $20 I spent was on a Radio Shack timer. I now have 2.

Alarm Clocks, Zen Alarm Clock, Progression Wake Up Clock, The Sonic Grenade
Cell Phone Alarms
PDA's i.e., Palm, Crackberry
Watch Alarm, Special Watches

External Websites

Mac Based Email plugins and Addons Hawk Wings list "You will find 120+ plugins, add-ons, scripts and helpful apps here."