It will turn the list into a checkbox list.

(setq org-capture-templates-contexts
 '(("r" ((in-mode . "gnus-summary-mode")
     (in-mode . "gnus-article-mode")
     (in-mode . "message-mode")))))

Here the capture template associated with the r key will only be displayed in gnus-summary-mode, gnus-article-mode and message-mode.

Quoting the Org manual:

The agenda* view is the same as agenda except that it only considers appointments, i.e., scheduled and deadline items that have a time specification [h]h:mm in their time-stamps.

(setq org-agenda-custom-commands
 '(("%" "Appointments" agenda* "Today's appointments"
     ((org-agenda-span 1)
 (org-agenda-max-entries 3)))))

See the example above.

Very useful when defining agenda views dedicated to appointments.

"Here is another neat trick. The sample code above also sets org-agenda-max-entries:

(setq org-agenda-custom-commands
 '(("%" "Appointments" agenda* "Today's appointments"
     ((org-agenda-span 1)
 (org-agenda-max-entries 3)))))

The use of org-agenda-max-entries will limit the agenda view to the first three entries of today's tasks.

C-h v org-agenda-max-entries RET to know more about this variable.

It will store the displayed list of tasks in a new agenda.org file.

Very useful to (export and) share your current list of tasks with someone.

Hitting = in the agenda buffer will limit to headlines matching a regular expression.

You can remove all filters by hitting |.

After C-c C-s on a headline scheduled in the past or the future, use . to jump to today's date.

After you hit C-c C-s on a headline to schedule it, enter e.g. +3d to plan in three days, or 10:00+1 to plan an meeting from 10am to 11am.

It will move the current cell up/down/left/right.

| Numbers | Dates |
|---------+-------------------|
| 1 | <2020-02-04 mar.> |
| 2 | <2020-02-05 mer.> |
| | |

Hitting S-RET here will create a new row and a cell with "3".

Hitting S-RET there will create a new row and a cell with "<2020-02-06 Thu>"

Org tables are able to sum durations:

| 00:00:30 | 2:30 |
| 00:00:45 | 3:30 |
|----------+------|
| 00:01:15 | 6.00 |
#+TBLFM: @3$1=vsum(@1..@2);T::@3$2=vsum(@1..@-1);t

When org-use-speed-commands is set to t, hitting s at the beginning of a headline will narrow to the current subtree. Hitting s again will unnarrow the buffer.

Very useful when you need to hide the rest of your personal notes from your colleagues.

The related command is org-narrow-to-subtree.

When org-use-speed-commands is set to t, hitting < at the beginning of a headline will activate the restriction lock on the current subtree. Hitting < again will deactivate it.

Very useful when you temporarily need to call agenda views while only considering tasks in the current subtree or when you need to undo things in the current subtree alone.

The related command is org-agenda-set-restriction-lock.

C-c C-^ (or C-c C-u) while move the cursor up one element.

C-c - on a headline will convert it into a list item.

C-c * on a list item will convert it into a headline.

You can also try on multiple headlines and list items.

- 0:00:00 :: Now I can start taking some notes.
- 0:00:02 :: If I hit ~M-RET~, a new list item is created.
- 0:00:06 :: And so on.

C-c C-x - will start a timer and create a list item displaying the timer like the example above.

When org-loop-over-headlines-in-active-region is set to t and the region is active on multiple headline, you can use C-c C-s to schedule all the headlines one after another.

See the docstring of org-loop-over-headlines-in-active-region for more fun.

M-h will mark (and highlight) the paragraph at point.

In org-mode, M-h will mark (and highlight) the element at point.

Hitting M-h several times successively will mark the next elements too.

Just hit M-<up/down> in an Org buffer. It moves elements.

Now hit S-M-: it moves the current line up/down.

If a property can have multiple predefined values and the column view is active, you can use S-<left/right> in column mode to cycle through possible property values.

I use something like this in my configuration:

(setq org-tag-alist
 '((:startgroup)
    ("Handson" . ?o) ;; The grouping tag
    (:grouptags)
    ("Write" . ?w) ("Code" . ?c) ("Mail" . ?@)
    (:endgroup)
    (:startgroup)
    ("Handsoff" . ?f) ;; The grouping tag
    (:grouptags)
    ("Read" . ?r) ("View" . ?v) ("Listen" . ?l)
    (:endgroup)
    ;; Off groups
    ("Print" . ?P) ("Buy" . ?B) ("Patch" . ?p) ("Bug" . ?b)))

Then M-x org-agenda RET m +Handsoff will list all headlines with a Read, View or Listen tag.

• S-up/down on a timestamp in a CLOCK: line will adjust the timestamp.
• C-S-up/down on a timestamp in a CLOCK: line will adjust both timestamp, the time you clocked in and the time you clocked out: the clock duration will not change.
• M-S-up/down on a timestamp in a CLOCK: line will also adjust previous clocks timestamps, if any.

C-u C-u C-c c will jump to the last captured note. The one you may actually want to update if you forgot something.

This add an :ARCHIVE: tag to the current entry, making it unavailable for most commands like (un)folding, agenda views, etc. It is bound to C-c C-x a - less known than org-archive-subtree, but useful too.

M-x org-num-mode RET will prefix headlines with a number.

...

Thanks to Karl Voit and Steven Shaw for their feedback and suggestions.