Is Mozilla taking a run at Outlook? Sure looks like it based on this page at Mozilla:
Welcome to the Mozilla Calendar Project. We are striving to build a cross-platform fully standards based calendar client based on the open iCal standard. Our client is built using the XUL user interface language and is targeted at Mozilla based browsers, including the Netscape 7.x series of browsers, Mozilla Firefox, Mozilla Thunderbird and the Mozilla Application Suite.
So pair up a highly functional Sunbird calendar built on an open standard with an awesome mail program like Thunderbird (by the way, you don't have to store your Thunderbird e-mails in goofy PST files) and you wind up with a pretty powerful competitor to Outlook. To that end:
Mozilla developer Mike Shaver has announced the "Lightning project". "Lightning" is the working project name for an extension to tightly integrate calendar functionality (scheduling, tasks, etc.) into Thunderbird.
With Lightning, Mozilla Thunderbird will have a set of user features that is much more competitive with Outlook, especially in enterprise usage.
It's hard to beat the price of Mozilla products (ummm, free).