Office dwellers, what software/website or system do you use to keep track of tasks you need to do?

Nothing worked for me until I designed my own planner. I like to take things one week at a time so every Friday afternoon, I print out enough sheets for the next week on semi-A4 paper, folded and stapled to a semi-A5 booklet.

One full page for each day with:

• Compact visual schedule of the day with a time grid (hours on the y-axis, 10s of minutes on the x-axis) and recurring events pre-printed
• "Today" box to write down reminders and tasks that don't go on a time grid
• Section to jot down miscellaneous thoughts and ideas
• Right half of the page entirely for a journal entry

Front cover has the weekly overview and back cover has upcoming and assorted tasks.

No monthly calendar, any entry that needs to persist for longer than a week or so goes in a separate hardcover A5 journal that is usually in my bag.