Time Machine
Keeps track of every version of every file on your computer automatically. The idea is that you'll never accidentally delete a home movie, or overwrite a report. If you do, just go "back in time" and restore the file. Restoration is done in a pretty swank manner by dropping the desktop down so the background becomes this field of stars. Then you traverse the timeline and find the right version of the folder or file to restore. The reasoning behind this feature is that everybody is always told to keep backups, but very few people actually do. One potential worry of this feature is how much computer resources will be used up, and is your hard drive going to get quickly filled? Probably not, but we'll see.
Mail
The mail program has been drastically updated. Now todo lists and notes are integrated into the system, just like in Outlook. The coolest addition is the use of e-mail templates. With these templates people can create really great looking e-mails very quickly, incorporating their photos in a way that makes sense and looks fantastic.
Spaces
Just a virtual desktop manager. These have been around for a long time and are popular on Linux systems. For people that aren't familiar with these, you essentially get 4 different virtual desktops to work on. This is great when you have a lot of windows open, especially if they're unrelated. Maybe you have a few windows for relaxing (fun websites, IM, Sudoku), then you have other work related windows, then a bunch of e-mail/task windows. Before all of those would be grouped in one place and difficult to manage. Now you can put each set of windows in their own desktop so they're nice and organized.
iChat
How much can you really do with chat? I wouldn't have known until Apple showed it. The new chat integrates with all the major IM networks, and when talking with other iChat users a whole wealth of cool features opens up. One of the coolest things is the iChat Theatre which lets you give a virtual presentation showing pictures and video on one side of the screen, with the video chat user appearing on the other. It might not sound that impressive, but watch the video and it looks pretty darn sweet.
