PmWiki uses a number of directives to specify page titles, descriptions, page keywords, and control the display of various components. Directive keywords are not case sensitive, e.g. Description, description, and DESCRIPTION are equivalent.

Shows a list of attachments of the current group or page, depending on whether attachments are organised per group or per page. The attachlist is displayed at the foot of the uploads page form.
The parameter to (:attachlist:) always resolves to a pagename. The directive then displays all of the attachments currently available for that page.
(:attachlist NAME:) shows a list of attachments of the group or page NAME.
(:attachlist {*$Name}:) shows a list of attachments of the current page or group. (useful for making a Group Headers or Footers in a case of per-page attachments)
(:attachlist ext=xxx,yyy:) comma separated extension patterns for filtering by type of file (case insensitive).
(:attachlist names=*.jpg,*.pdf,-private*.jpg:) comma separated filename patterns for filtering by file name (case insensitive).
(:attachlist *:) shows the uploads directory and permits browsing of all uploaded files by directory if your server configuration allows automatic file listings (will not work if $EnableDirectDownload is set to 0).

(:description text:)
Descriptive text associated with the page. (Generates a <meta name='description' content='...' /> element in the page output.)

(:keywords word1, word2, ...:)
Identifies keywords associated with the page. These are not displayed anywhere, but are useful to help search engines locate the page. (Essentially, this generates a <meta name='keywords' content='...' /> element in the output.)

(:linebreaks:), (:nolinebreaks:)
Honors any newlines in the markup; i.e., text entered on separate lines in the markup will appear as separate lines in the output. Use (:nolinebreaks:) to cause text lines to automatically join again.

(:linkwikiwords:), (:nolinkwikiwords:)
Enables/disables WikiWord links in text. Note, this setting requires WikiWords to be enabled, see $EnableWikiWords. See also $LinkWikiWords.

(:markup:) ... (:markupend:) or (:markup:)[=...=]
Can be used for markup examples, showing first the markup and then the result of the markup.
(:markup class=horiz:) will show the markup side by side instead of one upon the other.
(:markup class=norender:) will show only the markup not the result.
(:markup caption='...':) adds a caption to the markup example.
(:markupend:) is not required when using (:markup:)[=...=].
Note that the placement of newlines is very important for this markup. If you are using the [=...=] option then the opening [= MUST occur on the same line as the (:markup:). If you are using the full (:markup:) ... (:markupend:) form then your markup code must appear AFTER a newline after the initial (:markup:).

Displays messages from PmWiki or recipes, for instance from editing pages.

Turns off the section of the skin marked by <!--PageActionFmt--> thru <!--/PageActionFmt-->. In the PmWiki skin, this turns off the display of the actions at the top-right of the page (other skins may locate the actions in other locations). The actions at the bottom of the page are still available.

Turns off any groupheader or groupfooter for the page. (See GroupHeaders.)

(:noheader:), (:nofooter:)
(:noleft:), (:noright:), (:notitle:)
If supported by the skin, each of these turns off the corresponding portion of the page.

(:redirect PageName:)
Redirects to another wiki page.
(:redirect PageName#anchor:)
Redirects to an anchor within a page
(:redirect PageName status=301 from=name quiet=1:)
Redirects the browser to another page, along with a redirect message. For security reasons this only redirects to other pages within the wiki and does not redirect to external urls. The status= option can be used to return a different HTTP status code as part of the redirect. The from= option limits redirects to occurring only on pages matching the wildcarded name (helpful when (:redirect:) is in another page). The quiet=1 option allows the target page not to display a link back to the original page ($EnableRedirectQuiet variable should be set to 1).

(:spacewikiwords:), (:nospacewikiwords:)
Enables/disables automatic spacing of WikiWords in text.

(:title text:)
Sets a page's title to be something other than the page's name. The title text can contain apostrophes and other special characters. If there are multiple titles in a page, the last one encountered wins (see also $EnablePageTitlePriority about how to change it).
(:toc:), (:notoc:)
Override the default table of contents settings. This directive requires a corresponding configuration of the wiki, see Table of contents Basic automatic table of contents and numbered headings

Sets a page's summary. Used in some of the default PageList Templates. Note, this is a PageTextVariable and unlike core page directives it is case sensitive.

Can I get (:redirect:) to return a "moved permanently" (HTTP 301) status code?

Use (:redirect PageName status=301:).

Is there any way to prevent the "redirected from" message from showing at the top of the target page when I use (:redirect:)?

From version 2.2.1 on, set in config.php $EnableRedirectQuiet=1; and in the page (:redirect OtherPage quiet=1:) for a quiet redirect.

Is there any method for redirecting to the equivalent page in a different group, i.e. from BadGroup/thispage => GoodGroup/thispage using similar markup to (:redirect Goodgroup.{Name}:)?

(:redirect Goodgroup.{$Name}:) works if you want to put it in one page.
If you want it to work for the entire group, put (:redirect Goodgroup.{*$Name}:) into Badgroup.GroupHeader - however, that only works with pages that really exist in Goodgroup; if you visit a page in Badgroup without a corresponding page of the same name in Goodgroup, instead of being redirected to a nonexistant page, you get the redirect Directive at the top of the page.
With (:if exists Goodgroup.{*$Name}:)(:redirect Goodgroup.{*$Name}:)(:ifend:) in Badgroup.GroupHeader you get redirected to Goodgroup.Name if it exists, otherwise you get Badgroup.Name without the bit of code displayed.

How can a wiki enable linebreaks by default, i.e. without having the directive (:linebreaks:) in a page or in a GroupHeader?

Add to config.php such a line:
$HTMLPNewline = '<br/>';

This page may have a more recent version on PmWiki:PageDirectives, and a talk page: PmWiki:PageDirectives-Talk.