Osmo, a compact yet feature-rich personal information organizer for Linux, separates itself from the pack of other calendar applications due to its light weight and easy-to-use design. The GPL-licensed Osmo includes a datebook calendar, a to-do organizer, and a contacts list, all with lots of intuitive options.
Osmo is so new that it isn't included in many distributions' software repositories yet, but the source is available through SourceForge.net. Installing from the source isn't difficult. The only dependencies are the GIMP Toolkit (GTK+) and libxml2, which come with most distributions.