Canon. What a funny, strange thing. Even Gene Roddenberry couldn't truly lock in what Star Trek canon was supposed to be as, depending on the day, he both included and didn't include the Animated Series. That actually leads to one of the entries on this list, though no spoilers yet.
The Star Trek novels have been going for almost as long as the on-screen media has been, with the first dropping back in 1967 - although it was panned by fans of the series, and critics alike. Thankfully, there is a huge variety when it comes to the expanded universe to choose from to enjoy. They are continually released, with some of the newer novels offering a deeper understanding of the latest in Trek timelines.
Some characters who showed up in the show, only to be forgotten, finally received their send-off in the novels. Others got more character development, while others finally got a homeworld, which is always nice. Fan theories have been confirmed (and quashed), while official explanations have been shunted to the side as well. Again, canon is often in the eye of the beholder in these stories. For the purposes of this list, these entries will be treated as canon, but who is to say that Roddenberry wouldn't just change his mind tomorrow anyway.
10. Trelane's True Nature - Q-Squared
Peter David's novel, Q-Squared, finally confirmed what audiences had suspected since Q's first appearance in Encounter At Farpoint, that Trelane really was a member of the Q continuum. This has become a generally accepted fact, even if this includes a small amount of retconning.
In The Squire Of Gothos, Trelane's power seems to come from a large, ornate mirror. Destroying said mirror helps Captain Kirk and his crew plot their escape before Trelane is taken away by his parents. However here, Trelane is revealed to be much, much more than they originally suspected.
The familiar Q, who was played by John de Lancie in the series, appears to Captain Picard in this novel, seeking the man's help in stopping Trelane. He has gained access to a device that gives him power far beyond that of the entire Continuum, leaving them out of options. The novel takes place in three different timelines, also retroactively 'fixing' the continuity gap created by James R. Kirk's headstone, as created by Gary Mitchell.
The audience may have assumed that Trelane was a Q all along, but it was David's novel that actually confirmed it.