[What details should go in here?]
[What and when was the first Tk extension?]
Changes in Tcl/Tk points to a series of pages that detail what features are added, modified, or deleted in each release.
http://www2.sdtimes.com/content/article.aspx?ArticleID=31644 is an article covering the 20th anniversary (birthday?) of Tcl.
[Explain the Great Re-Unification of version numbers.]
DKF 19-Feb-2008: From 8.0 onwards, it was decided (by JO/Scriptics) to make the version number of Tcl and Tk match. Without that, it's probably the case that Tk would still be in the 4.* version number series, as Tk apps from 4.0 onwards are highly-compatible with the current release.