I use www.Astrid.com. Started with their excellent free Open Source app for Android... I always have my phone so a good app for it was important. It's both well designed and feature rich. It syncs with Google tasks and producteev so I have sometimes used those on the web, but their website has gotten good enough that I generally just use it when I'm at computer.
It's in active development, integrates with the pomodoro technique, and has the right balance of features/ease of use that I need from a task list tool.
I have been around the block using these! Springpad, remember the milk, wunderlust, evernote, onenote, outlook tasks, I could go on and on.
i. I carry around a real notepad for quick notes. I fill up about 1 every 6 months, sometimes I take quick notes on iOS Notes app.
ii. Short term TODO's I email to myself and flag important.
iii. Daily stuff goes in Notepad or Sticky notes on my desktop, open all day, synced in dropbox.
iv. Long term stuff goes in Google Tasks under different categories, and relevant web links to those goes in it's corresponding folder in chrome, synced across browsers.
At work I use Outlook Tasks, because we use Outlook, Office and Sharepoint and it's easier to work within all that instead of fighting it.
Tasks are basically a customized Outlook message, so you can dump all manner of Office artifacts in a task to keep track of stuff. Copy anything from an Office or Outlook doc, or embed an entire document in a Task.
So the existence of the Task is part of my ToDo list, and what's in the Task object is stuff i've been keeping track of on a long running task. Simple tasks are just a Task with a title and a due date.
At work, I just use email. Whenever I check my inbox, I delete all emails that I no longer need or are irrelevant and I trim my inbox aggressively. All emails left there are things that I have to do. I recently switched to this method after having an overflowing inbox with 30 folders and all kinds of filters, etc, and I'm already seeing a huge productivity increase. Still, I fully appreciate this won't work for everyone, especially those who are getting 100s of emails per hour.
OmniFocus: I love the way I can assign a todo to location context and have it pop up automatically when I next visit that place.
In the end, the app or site doesn't matter, it just matters that you have one list that is always available. I'm still looking for a waterproof for the only exception to the single list.
I used Review19 -- http://review19.com (Disclaimer: this is my app) for collaborative project tracking and that covers my consulting work and personal projects.
I've used Trello and Google Docs in the past and they work great as well.
Remember the milk. The great mobile app keeps everything in sync. I use a vim text file with outline style folding for my project list and notes, GTD style. I sync that with git on github so that I have history without clutter and read access everywhere if needed.
