The secret tool here is Gmail or Google Apps. This will be the glue that ties all of these devices together, and will be used as the target for the synchronizations. Hence you will need a Gmail or Google Apps account. Before we begin, there are a few things we need to do:
Export your contact and calendar data.
Your setup may look very different, but the bottom line is that you need to export all contact and calendar data from your various applications. If you're on a Mac, chances are you use iCal for your calendar, and Address Book for contacts. Also make sure you sync all your iOS-devices (iPhone/iPad/iPod) so that the contact/calendar data on your iCal/Address Book is up to date. To export your contacts from Address Book, select all your contacts, go to File -> Export -> Export vCard. To export your calendar entries from iCal, select the calendar, go to File -> Export -> Export. Repeat if you have multiple calendars. Make sure you save these files somewhere safe. If you're not using any of these applications, please consult the manual for instructions on how to export the contact and calendar data.
Import your contact and calendar data.
Now you need to import the data into Gmail and Google Calendar. Here's video on how to export contacts from Yahoo and import into Gmail. You can skip the first part, but the import part is the same. This video goes through the same process but for Calendar data. Again, just skip the first section. The import part is the same.
Validate the data
Take some time to validate the import. Make sure all the contacts and calendars are there.
Mac OS X
The key to set up iCal is to make sure you use CalDAV, and not just subscribe to your calendars. The problem with if you just 'subscribe' to your Google Calendar is that you cannot edit it (ie. add/edit events). Obviously, being able to edit your calendar is key. Setting this up is rather simple, and you'll find the instructions here.
Setting up the Address Book is even easier. The key here is to start with a clean address book to avoid conflicts. Since you already exported your contacts and imported them into your Google accounts, you can delete all contacts. Also, make sure you save that export you did before as a backup. Once you have an empty address book, just go to File -> Preferences -> Accounts. Then click 'Synchronize with Google' and press 'Configure.' The wizard should take you through the steps and you should have your contacts synced into your address book shortly.
The only thing you need to keep in mind when setting up your email in Apple Mail is to use IMAP (details on how to do that here). If you have your email set up using POP3, you need to add the account as IMAP. Then drag all the existing emails over to the IMAP account, as the existing email (if you're using POP3) only resides on your local computer, which makes it impossible to stay sync'ed across devices.
Not much explanation is required for Android. Recent versions of Android, automatically syncs up with Gmail/Google Apps for email, contacts and calendar.
iOS devices (iPad, iPhone, iPod Touch)
This is the most common problem I hear people complaining about. They think they need Mobile Me to sync contact and calendar items with their iOS device. That's wrong. You can do that with Google Apps or Gmail. It just requires some special configuration. Luckily, yours truly is here to help. Just follow this guide, and you should be on your way to a cable-less life. That's it. If you followed the steps above, you should never have to sync your devices with a cable again (at least not for contacts and calendar data).