Switching: iPhone to Android (Contacts)

First and foremost, you need contacts.  You certainly don't want to actually remember the phone number and email address of every person you know or have met in passing.  That's why smartphones were originally invented!  No, you probably have a whole slew of contacts stored in your iPhone.

So, how do you get your contacts out?

Unfortunately, the answer to that question is situational.  There are several ways to manage your contacts on an iPhone:  natively;  via Exchange;  via third party providers like Google or Yahoo.  If you're already using Google to manage your contacts on your iPhone, well, you're done.  For every other option there's a bit of elbow grease involved.

Mail account type selection screen

If You Already Use Your Google Account on Your iPhone
It is important to note that there are two ways to set up your mail for Gmail on your iPhone, but only one of which allows you to sync your contacts.

If you selected "Gmail" when creating your email account on your iPhone, this does not sync your contacts with Google.  You are also not getting "push" mail.  You're only checking the server every 15 minutes or so for your email.  If you created your Gmail mail account this way, then your contacts are being managed by other means.

Example Google Account via Exchange

However, if you get push mail, then you set up your Gmail account through the "Microsoft Exchange" option.  This will give you the ability to sync your contacts.

You can check if you are syncing your contacts by going to Settings -> "Mail, Contacts, Calendars" and choosing your Gmail account (it will be named yourusername@gmail.com, most likely).  Choose that account, and if Contacts is set to "on" (as pictured on the right), then your contacts are already synced with your Google account.

Pro Tip:  If Google is managing your contacts, then all of your iPhone contacts will appear in your "My Contacts" group in your Gmail account.  When you switch to Android, your Android phone will pull not only this group, but all of your contacts in Gmail, so you may end up with more contacts in Android than you have on the iPhone.

Native:  Via iTunes
If you use the iPhone Contacts app to natively manage your contacts, then they will be synced up with the iTunes profile for your phone.  Syncing your contacts into your Google account is rather easy, once you know what to do.  The basic gist is that there is an option in iTunes to sync your contacts to your Google account.

Here are the how-tos from Google:

For PC
For Mac

Via Exchange
If you use Exchange to manage your contacts, email and calendar, then it's very likely that you are an enterprise user.  There's no easy and straight forward way to go from Exchange to your Google account, but there is a tried-and-true method for contacts:  the CSV file.

The short answer:  Export your contacts from Outlook into a CSV (comma separated value) file, then import that CSV file into Gmail.

There are literally hundreds of sites which can tell you how to do each of these steps, but here's an article on c|net which walks you through the process from start to finish on Outlook 2003 and 2007.  The article is from 2008 (meaning the Gmail import may be slightly different now-a-days), but should demonstrate the export process well enough:

Exporting contacts from Outlook to Gmail

Here's the Google help page for importing a CSV file into Gmail:

Importing a CSV file into Gmail

Pro Tip:  Even if your iPhone is using an Exchange account for your contacts, it doesn't necessarily mean that you're connecting to an Exchange server.  A lot of third party providers, for instance Google, use the Exchange format as it is a standard way to sync email, contacts and calendars on the iPhone.

Via Third Party Providers
If you use a third party provider other than Google to manage your contacts on your iPhone, then you will need to look up how to move from your third party account to your Google account.  Two of the big ones are Yahoo and Hotmail.  Keep in mind that this is more of a question of moving your contacts from your current provider to your Google account, not necessarily from your iPhone directly.

For moving contacts from Yahoo to your Google account, look here:

Import Yahoo contacts into Gmail (when you export, export for Microsoft Outlook, then follow the above instructions for importing a CSV file into Gmail)

To move from Hotmail to Google, look here:

Google Support on Moving from Hotmail to Gmail

For any other providers, you will need to look them up individually.  A useful Google search is "export contacts from XXXXXX into Gmail", where XXXXXX is the provider you're using.