A new post on the Catznip site announces the coming Spell Check and Auto-Replace features coming to Linden Lab’s Beta Viewer. The LL version is from Kitty Barnett and I think first seen in Catznip R6, the current release of Catznip.
In the back-and-forth development to submit the code to the Lab the features have undergone some change. The Catznip Team considers this to be one of the largest third party submissions via the Contributor Agreement process.
See: Catznip Spellchecker Official LL Beta.