(Via Ed Brill)
Lots of people are talking about what new competitors to Microsoft's Outlook e-mail programs, from desktop applications like Thunderbird to web-based programs like Zimbra or Google Mail. But in fact, there's only one program that really comes close to challenging Outlook today.
That program is Lotus Notes.
See the article (or the many comments in Ed's blog post referencing the article) for more details/discussion/debate...
I think there's more to the story, however: if you consider the Eclipse.org-based foundation in Notes 8, along with the OpenOffice.org-derived Lotus Symphony "productivity editors", the fact that Notes 8 runs on multiple platforms (eventually multiple versions of Windows, Linux, and Mac OS), and the fact that Notes is also a template-driven application development platform (with a tightly-coupled and purpose-built, multi-platform server counterpart), the competitive overlap goes far beyond Outlook. You could argue that aspects of Notes actually compete with Windows, Office (not just Outlook), and even the .NET Framework.
It's also true that Notes is the leading competitor to Outlook, of course, and Outlook's role has expanded quite a bit in Office 2007 -- e.g., for forms-based applications, serving as the primary off-line-able Microsoft solution for SharePoint document libraries and discussions, as Microsoft's primary XML syndication client, and more.
So in many respects it's business as usual, with IBM Lotus and Microsoft competing for the privilege of being suppliers of the application where information workers spend most of their time each day. Both IBM and Microsoft are hedging their bets, e.g., with IBM promoting Quickr and Connections, especially to customers/prospects who (for whatever reason) don't want Notes, and Microsoft also emphasizing Groove and browser-based SharePoint capabilities, but at the end of the day most information workers close either Notes or Outlook before they head home (often only to re-open the applications after dinner...).