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iSync
iSync is a very useful program put out by Apple to sync up Address Book and iCal between external sources and computers. These include Plam handhelds, iPod, bluetooth enabled phones (if you have a bluetooth module), and a .Mac account. Its a really simple program to use and if you have any gadgets to sync up with you should.
Add a Device
Open up iSync, it should be in your Applications folder on your Hard Drive. If you don't have it you can download it using Software Update. Once the program is open go up to File and down to Add Device. A new window will open and it may or may not show the devices you have hooked up (if they're not plugged in to your computer, or within range of bluetooth, your computer will not find them). If you know you have stuff hooked up correctly and stuff doesn't show up, hit the Scan button. Double click on the device when it shows up to add it to iSync. If you're trying to sync up a bluetooth phone, or even a bluetooth PDA, make sure that Bluetooth is on in both your device and your computer. also be sure to have the device near your computer,s eeing how bluetooth has a very short range. Also, bluetooth is kinda slow at times, so when syncing be sure to give it some time.

(They're all playing nice together)
<Syncing
Once you have added all your devices you want to tell iSync what to do with each of them. Click on the device you want to configure. The window should expand giving you a set of options. In the picture below I'm syncing up my iPod with my Mac.

First off if you want the device to sync you need to click on the first box. If you wish to have iSync automaticly sync that device everytime you connect it to your computer, check the second box. The Contacts area is what syncs up Address Book with your device. You can choose to sync all your contacts or only certain groups. The Calendars part is for use with iCal. You can sync all your calendars or only certain ones.
If you have a .Mac account (you'd have had to pay some money for it) you can use iSync to sync up more than one computer via the internet. Such as between your work computer and your home computer. I don't have a .mac account so I don't know how wellit works.
Syncing
Once all your settings are in place you can sync your stuff. Just click on the big Sync Now button in the top right of the window and all your stuff will be synced! Your contacts and calendars will now be on your devices for your viewing peasure!
I have only used iSync with my iPod. My roommates have used it with Palm Handhelds and it works great also. Its a nice replacement for Palm Desktop, though you need to keep the Palm Desktop program on your Mac to use your handheld with iSync, because iSync installs a plug-in into Palm Desktop that allows it to function. If you have an iPod or PDA I highly recomend that you use iSync to keep your contacts organized.
-Joshua