From time to time, TYPO3 core devs and insiders leak secret knowledge in the mailinglists. One can find questions about "how to do this?" with answers surprisingly revealing undocumented features. Read on for some hints about how to find those features without asking.
TYPO3 wiki: pending documentation section
The first place to look for those features is the pending documentation section in the TYPO3 wiki. It's meant to be the place for documentation of all new features, which have been added to the trunk, but not yet released. The items are collected here, reviewed and then put into the docs at typo3.org. Sometimes, this process is a little bit delayed and the wiki page becomes the only referrer to new features of an actual release. In this context, we can see Dmitry bashing Michael in the core list about that: "(...) Stucki is responsible for [not] updating core documentation in time. kick him, I am already tired of doing it. (...)" (Dmitry, 2008/07/31).
Francois kindly updated TSref with all pending parts from the wiki. To be release soon...
Bugtracker: documentation category
Another place is the bugtracker. There is a documentation section in mantis, where tough users could have reported items about missing documentation.
Mailinglists: recently committed features
Last but not least, you could RTFML, especially the core list. Use the search function of support.typo3.org for a full bodytext search. Although "we cannot expect from our users to read the Core list to learn about new features" (Masi, 2008/08/04), it however seems to be the price one has to pay for living on the bleeding edge.