Wild Bill Simser has a most interesting post about 'The Sad State of Groove add-ons'. I couldn't agree more with Bill. Seeing Groove development for nearly 5 years now and much has changed. The Groove Tools SDK is very much missed by the Groove development community. There has not been much Groove tools (these add-on to the Groove space itself) over the years but the few that were, were quite exceptional. A notable example is CompuTact-Apwiz (a company that I am associated with) - over the past 5 years we have had more tools than Groove Networks itself! As mentioned by Bill - other tools came from Information Patterns & NetSender. There were others (TeamDirection another exceptional one). Groove space add-ons were a very innovative (& much loved) way of enhancing the Groove client. The Groove add-on SDK (with the Visual Studio integration) had won the famed Jolt Award (& Sam Gentile was part of that effort.. :-)
The Groove development model has changed considerably - it is now with Groove Web Services, which is used with your external application (not an add-on) to communicate with the Groove client. The Groove development community has already leveraged this model (Dicodemy has a portfolio of tools already released).
There is another facet of Groove that will be sorely missed - the Groove Mobile Workspace where a Groove Space and a WSS SharePoint site were under one umbrella. This model was pursued in the Tsunami Relief efforts (a Virtual Medical Village to communicate with non-Groove clients). The new version of Groove 2007 is now relegated only to SharePoint document file-sharing albeit Groove 2007 is in beta, it remains to be seen what it will be in the release version. All said and done, Groove still provides a great collaborative platform that is greatly secure.