Today from Second Gear Software is a lightweight calendar app for the Mac that focuses on simplicity and speed. It's not an alternative to iCal, but rather a tidy front end. With some configuration, you'll be using Today to create events, tasks and more, all with custom keyboard shortcuts. I've spent a few days using Today. Here is my review.
Note: yesterday I reviewed Fantastical from Flexibits, and tomorrow I'll post a head-to-head shootout between the two. Don't miss it. Additionally, I'll have a brief bonus comparison to Quick Cal