As you might be able to guess, I am fond of the Gospel of Thomas.
This one was lost to history until roughly fifty years ago a complete codex was found in Egypt with the Nag Hammadi Library. Until the discovery, we only had fragments in ancient Greek.
Unlike the Synoptic Gospels, John's Gospel and other apocryphal gospels, Thomas is not in narrative form. Instead, Thomas is a 'sayings gospel' - a compilation of sayings attributed to Jesus.
In my opinion, Thomas theologically coincides with the Synoptic Gospels more so than John coincides with the Synoptic Gospels. Thomas was most certainly written prior to John and may pre-date the Synoptic Gospels as well. A few scholars have even proposed that Thomas is the Q document, though such a claim is impossible to prove.
Xris - that Jesus had siblings is not unique to Thomas. Mathew, Mark and Galatians all contain mentions of Jesus' family with words often translated as brother and sister. As is so common, the question hinges on translation.
As for Thomas being kept out of official cannon, this is a long and complex story. We have no definitive answer despite the common claim that Thomas has disagreements with the other gospels. Having read the other Gospels, I find that claim to be rather silly given that John has mountains of discrepancies with the Synoptic Gospels to the point where there almost seems to be two Jesus - one of the Synoptic Gospels and one of John.
The more likely reason(s) for Thomas to be excluded are that Thomas contains hints of esotericism, was used by gnostics, and more than any other reason, presents a Jesus who would disagree with central religious authority. The Thomasines simply did not wield the kind of political power held by other early Christian groups and leaders (such as the Bishops in Alexandria).