If your needs are basic there's Sunbird [1]. It's a small, resource efficient calendaring application and has a really neat feature that displays your to-do list for the day on your desktop . However it really is a bit minimal. Users of the Thunderbird email client would be better off with the Lightning extension [2] rather than Sunbird, as it integrates calendaring features directly into the Thunderbird interface.
If you want more features, check out EssentialPIM [3]. It does many of the same things as Outlook and can import your Outlook data as well.
Yet another alternative is the PalmOne Desktop program [4]. The program is designed specifically for use with Palm devices but it works perfectly well all by itself. It's an excellent full function PIM; if you think Microsoft Outlook without the email client you'll get the general idea. I only did a quick check-out rather than a full review but what I saw seemed pretty solid. Naturally a few things such as "Synchronize" don't work without an attached Palm device but most functions work just fine. I also checked the licensing agreement and while it's a little ambiguous I suspect its OK for personal use. Indeed, I can't see Palm objecting as it may well encourage you to buy one of their hand-helds. ;>)
[1] Sunbird
Website: http://www.mozilla.org/projects/calendar/sunbird.html
License: Free Open Source
Download file size: 5.8MB
Windows Operating Systems Supported: NT/2000/XP/2003 Server/Vista
Non-Windows Operating Systems Supported: None
64 Bit Capable: No
Portable Version Available: No
Additional Software Required: None
[2] Lightning
Website: http://www.mozilla.org/projects/calendar/lightning/
License: Freeware
Download file size:
Windows Operating Systems Supported: 95/98/ME/NT/2000/XP/Vista
Non-Windows Operating Systems Supported: None
64 Bit Capable: No
Portable Version Available: No
Additional Software Required: Mozilla Thunderbird
[3] EssentialPIM
Website: http://www.essentialpim.com/
License: Freeware
Download file size: 1.2MB
Windows Operating Systems Supported: 95/98/Me/NT/2000/XP/Vista
Non-Windows Operating Systems Supported: None
64 Bit Capable: No
Portable Version Available: No
Additional Software Required: None
[4] PalmOne Desktop
Website: http://www.palm.com/us/software/desktop/
License: Free for personal use
Download file size: 42MB
Windows Operating Systems Supported: NT SP6/2000/XP/Vista
Non-Windows Operating Systems Supported: Mac OS X
64 Bit Capable: No
Portable Version Available: No
Additional Software Required: None