Mozilla has released a 0.1 version of Lighting, a calendar extension for it's mail client, Thunderbird. A Calendar component already exists, as well as a standalone product called Sunbird, so I'm not sure I entirely understand why everyone is so excited.
I don't think that Lightning really a new product line, more of an evolution from the previous products, but it raises an interesting debate about which sort of calendar is better - online or offline. Online calendars like 30boxes have a few advantages. Firstly, they are easily accessable from multiple locations and ,potentially, devices. As they are on a webserver they can be accessed from any computer connected to an internet, and making them viewable on a mobile phone or PDA is as easy as providing an alternative stylesheet. They are also probably better suited to sharing calendars, as friends' online calendars can be synchronised and updated from each other easily.
However, offline calendars also have advantages. Despite recent advances in web technology (eg AJAX) offline UIs can be more complex and 'slicker' than their online counterparts. The look and feel of the calendar is not constrained by the browser rendering engine, and remains constant across one operating system. They are also likely to have more features as the architecture to provide them can be simpler and requires no ongoing cost to the developer.
The two options need not be mutually exclusive. With syncronisation feeds being built into more calendar tools and offering more power, there is no particular reason not to have an online calendar which feeds to an offline calendar. Then you get the best of both worlds.
[Maybe I'll do a review at some point, but in the meantime, 30boxes is an excellent calendar tool and offers a whole load of the syncronisation features I was talking about]