It's hard to find a phone nowadays that doesn't have a calendar, a few games, or even downloadable applications. But some phones go the extra mile to bring voice and data needs together, including syncing e-mail, surfing the Web, shooting pictures (Find out how we test phone cameras and see a slide show of results), and supporting hundreds of third-party apps. Some have full keyboards to get down and dirty with your e-mail. We call these multitalented devices smart phones.

If you're the type who craves 24/7 connectivity, a smart phone could be your answer. With one device, you can take calls, receive all your corporate and personal e-mail, and store your PIM information, including phone numbers, contacts, addresses, calendar, and to-do lists. And with the hundreds of apps available for smart phones, you can perform business tasks, like reading PDFs and Excel spreadsheets.

But smart phones aren't all business. Thanks to more powerful processors and more flexible OSs, many of these phones can play 3D games and MP3s, record and play back video, and bring together SMS and e-mail.

Many smart phones come with freely extensible OSs that can host third-party applications. These apps let you expand a phone's capabilities—giving it true smarts.

BlackBerry OS is extremely e-mail focused. BlackBerry devices hook up to corporate or Web-based e-mail servers for up-to-the-minute e-mail. (You can do this with third-party software on other smart phones.) It's also very easy to read and send e-mail on any BlackBerry. But only a few third-party applications are available to enhance the OS.

Microsoft Windows Mobile for Smartphone (which we'll call Windows Smartphone in this story) allows you to sync with PCs and Microsoft Exchange servers and offers an e-mail application similar to Outlook. There are three versions of Windows Smartphone: the first-released Smartphone 2002; the considerably faster Smartphone 2003; and Smartphone 2003 Second Edition (which adds support for higher-resolution screens).

Palm OS is best known for its easy-to-use PIM applications, stylus-based input method, and vast array of third-party apps. Another advantage of Palm OS is that it's already been test driven for years on PDAs.

Some phones opt for proprietary operating systems, with few or no third-party applications. Many of these have keyboards, which make them good for sending and receiving SMS and e-mail.

Symbian is perhaps the least known among U.S. consumers, yet it is Europe's most popular smart-phone OS. It comes in various flavors, but all the Symbian phones we tested use the Series 60 version. Symbian uses much less memory and battery power than Windows Smartphone, the other major phone-only OS represented in this roundup.

As with Palm or Windows, third-party Symbian applications do everything from read Microsoft Office documents to help you plan routes through unfamiliar cities.

When it comes to carriers, you're limited by where you live, work, and travel, as well as whatever contract you're locked into.

U.S. carriers primarily use three cellular technologies: GSM (AT&T; Wireless, Cingular Wireless, and T-Mobile), CDMA (Sprint PCS and Verizon Wireless), and iDen (Nextel). GSM phones can also be sold unlocked, ready to use on any of the three GSM carriers.

Smart phones are still in their infancy, and it takes patience from early adopters to tap their power. But you'll be surprised at what you can accomplish with these gadgets. They may even drag you away from your good old PDA.

Our contributors: Associate editor Jenn DeFeo and lead analyst Sascha Segan were in charge of this story.