sgmllib [done]
* Does not fully parse SGML.
* In the stdlib for support to htmllib which is slated for removal.
Based on that and the standard docs, it looks like it was lost in the standard library reorganization. The HTMLParser and other HTML-related libraries were merged into a new html module; sgmllib's parser was an incomplete implementation of SGML that only HTMLParser used, so apparently in the reorg it was found unnecessary and scrapped.
But, if it worked for BeautifulSoup, as it existed in Python 2.5 would be suitable for BeautifulSoup to continue as if it were the last good version.... I'm not suggesting it be readded to the standard library, just that it be added to BeautifulSoup.