There's a lot of debate about the growth and share of the various collaboration platforms in the marketplace. The Wikipedia link lists notable vendors and products:
Vendors that offer collaboration platforms include Advancing Insights (enterprise social software), IBM (Lotus Notes/Domino, QuickPlace, and Sametime), Microsoft (Outlook/Exchange, SharePoint, Groove, Live Meeting, and Live Communications Server / Office Communicator), QXSystems (WorkACE), Novell (GroupWise, Teaming + Conferencing), Oracle (Oracle Collaboration Suite), Collanos Software (Collanos Workplace) and Sun Microsystems (Sun Java Enterprise System). An Open-Source teamwork collaboration solution is Mindquarry.
From my experience, IBM (Lotus Notes/Domino), Microsoft (SharePoint, Exchange, etc), Oracle and Novell are seen as the traditional leaders.
Market share has been based on analysts' estimates of vendors' claims. The usual measure is "seats," which means, roughly, unique users. Each vendor's products are licensed and reported differently, though, and vendors are not motivated to understate their sales. As a result, analysts' estimates are infrequent, imprecise, and often create more argument than insight. In addition, they reflect only a snapshot in time, so it takes successive estimates (over a number of years), to show growth or decline.
A better measure of a platform's share and growth would be how many projects are being built on it, compared to other platforms. After all, a collaboration platform is a computing platform -- something on which applications or solutions are built. Stagnant or low-share platforms would have the fewest projects, and the most popular and active would have the most.
By chance, I happened on IT Jobs Watch, a UK site whose aim is to "... to present a concise and accurate map of the prevailing UK IT job market conditions." This section of the site looks at the demand for Project Management skills across the UK (3 months to July 20, 2007).
I circled on the screen shot, the products that Wikipedia listed as collaboration platforms. Only two vendors were noted: IBM/Lotus (98) and Microsoft (337 projects). This supports the traditional view that Lotus and Microsoft lead this market. It also shows that Microsoft has a significant lead in active projects. Does this correlate to share, or growth?
There are obvious caveats: it's one country only, it's one point in time, it doesn't differentiate between projects of different sizes and complexity, and so on.
Despite that, this could be a valuable and objective index of platform share and growth and is worth watching over the long term.