People with multiple email accounts make frequent use of programs like Microsoft Outlook.
A common mistake among non-savvy emailers is that Outlook, or other similar programs, are email services. THIS IS NOT TRUE! These programs are merely email clients, or email organizers, and merely give a way to structure multiple email accounts.
Another problem is that Microsoft Outlook costs money. But there are free alternatives. Here are some free alternatives for Google App Email (there are other free email clients but many are not support by Google):
Windows Live Mail (Windows Vista & 7 only)
Outlook Express was replaced by Windows Mail, and then by Windows Live Mail. Those still using Outlook Express will need to upgrade when using Windows 7 since it is no longer supported.
You can still install Outlook Express (if you can find it) on Windows Vista or XP. However, it is quite a hassle for all the steps it would require to put it on. It will soon become fully unsupported in the future, so it is best to start using Windows Live Mail.
Mozilla Thunderbird (for both Mac & Windows)
Thunderbird is a project that Mozilla had started. They have since discontinued official support, however, support still exists through the community, and because the project is open source, there are hundreds of people working on evolving and updating this program.
Thunderbird also serves as a web browser (modeled after Firebird) and a IRC client, in addition to being an email client.
Apple Mail (for Macs only)
Apple Mail is a built-in mail client for all Macs with OSX 10.3 or beyond. OSX Lion brought about Apple Mail 5.0.
This is probably the easiest free setup available for Macs.
It should be noted that at the time of publishing this article, Google has not posted any instructions on Apple Mail 5.0, just Apple Mail 3.0 & 4.0. However, Google has also neglected to post any official instructions for Outlook 2010, yet you can find articles online as to how to set it up successfully.