This page is an unorganized attempt to document some features. General information about the site is over here.
On thread and message pages, the left and right arrow keys can be used to navigate to newer/older threads.
Each list is available as both an Atom 1.0 and an RSS 2.0 feed, but in this case Atom is preferred for its additional updated property: we use this to report the time of the most recent update to a thread. Appending a number to the end of the feed URL causes only that number of threads to be displayed, up to a limit. For example, the URL http://web.archiveorange.com/atom/list/5H3d1/1 produces an Atom feed containing only the most recent thread of the list python-list.python.org.
Messages that we have identified as missing are marked by placeholders in the thread. These messages may have been caught in a spam filter somewhere, they may just be taking a long time to get here (it happens), or they may have opted out of archiving by setting the X-No-Archive header. In any case, when the message arrives it will replace the placeholder block. Links to the placeholder will continue to work after being replaced by the real message.
Due to one of GNU Mailman's enigmatic decisions, the placeholder is disabled on lists managed by it. Threading has been broken for ages by Mailman stripping and rewriting the Message-Id header in certain cases. Mailman devs have assured that this is not a bug, it's a feature.
Locating threads by date is simple: the dates are specified right in the URL of the page showing recent threads for each list, and roughly follow the Python "slice" syntax with dates represented in the normal UNIX way as the number of seconds since January 1, 1970. Appending the date range to the end of a list's "recent threads" page displays only the threads in the range.
This is best illustrated by an example: I'm interested in threads in python-list.python.org around December 10th, 2009 at 18:44:11 GMT, which is 1260470651 seconds since the Unix epoch. Appending a date range of :1260470651 shows the 20 threads up to this time, 1260470651: shows 20 next threads after this time, and I can also specify ranges as StartTime:EndTime.