The latest iMac models were released on March 3, 2009.  They look very similar to the prior two generation models in that they maintain the aluminum enclosure and glossy glass display.  The aluminum iMac was first introduced in 2007.  Prior to the 2007 update, the all-in-one iMac enclosure was constructed of white plastic.  Here's a history of the all-in-one iMac models since August 2004.

Intel

• iMac (Early 2009) - March 3, 2009
  
• iMac (Early 2008) - April 28, 2008
• iMac (Mid 2007) - August 7, 2007
• iMac (Late 2006) - September 6, 2006
• iMac (Early 2006) - January 10, 2006

PowerPC

• iMac G5 (iSight) - October 12, 2005
• iMac G5 (Ambient Light Sensor) - May 3, 2005
• iMac G5 - August 31, 2004

In the Box

• iMac
• Power cord
• Apple Keyboard
• Mighty Mouse
• Install/restore DVDs
• Printed and electronic documentation

Key Configuration Options

• Processor up to 3.06GHz
• Memory (RAM) up to 8GB
• Hard drive up to 1 TB (7200-rpm)
• NVIDIA GeForce GT 130 (512MB memory)
• ATI Radeon HD 4850 (512MB memory)
• Apple USB Modem
• Apple Remote
• Apple Keyboard with Numeric Keypad
• Apple Wireless Keyboard
• Apple Wireless Mighty Mouse

Noteworthy features and changes in the Eary 2009 iMac models
The key improvements in the latest iMacs are directly related to processor, RAM, and graphics specifications.  Processor speeds have improved in conjunction with Apple implementing DDR3 RAM (memory) in the Early 2009 models versus the DDR2 installed in the Early 2008 models.  The following comparison highlights key features and changes when compared to the Early 2008 models:

Standard features:

• Slot-loading 8x SuperDrive (DVD±R DL/DVD±RW/CD-RW)
• One FireWire 800
• Four USB 2.0 ports; two ports available on wired keyboard (no USB ports on wireless keyboard option)
• Built-in iSight camera (at the top center of the display)
• Mini DisplayPort output - support for DVI, dual-link DVI, and VGA video (adapter required)
• Built-in AirPort Extreme 802.11n Wi-Fi wireless networking
  
• Built-in Bluetooth 2.1 + EDR (Enhanced Data Rate)
• Built-in 10/100/1000BASE-T Ethernet
• Built-in microphone
• Built-in stereo speakers
• Headphone/optical digital audio output (minijack)
• Audio line in/optical digital audio input (minijack)
• Built-in display and support for an external display
• Glossy widescreen TFT active-matrix liquid crystal display
• Supports extended desktop and video mirroring modes

Installed software
All iMac models ship pre-installed with:

• Mac OS X version 10.5 Leopard operating system which includes key features and applications such as Time Machine, Quick Look, Spaces, Spotlight, Dashboard, Mail, iChat, Safari, Address Book, QuickTime, iCal, DVD Player, Photo Booth, Xcode Developer Tools)
• iLife '09 which includes iPhoto, iMovie, iDVD, iWeb, and GarageBand

Design - All In One Display - Beautifully Aluminum
The iMac stands elegantly with a minimal desktop footprint.  The iMac ships with the Apple keyboard and wired Mightly Mouse.  If you decide to custom build your iMac (what Apple calls Configure-to-order), you can upgrade (for a fee) to the wireless keyboard and wireless Mighty Mouse.  The end result would be a iMac setup completely free of  desktop clutter. Although it's Apple's mid-range desktop, the iMac offers a full range of features and specifications to take advantage of today's digital lifestyle.  The iMac is easy to set up, just take it out of the box, place it on a desk, run the OS X setup assistant and you're off and running.

Eye Catching and Dazzling
Wow, does that look cool, there isn't a sexier desktop computer on the market.  Coolness aside, the iMacs are fun to work with and easy to use.  They feature a  widescreen  display perfect for watching movies but more importantly, they enhance productivity because you'll be able to see and do more at the same time.  With the built-in iSight camera centered at the top of the display, you'll have fun with video chats (iChat AV) and snapping pictures into applications such as Photo Booth.  No clunky web cams are needed.

Able to Meet Most Needs
Whether you're a casual user, prosumer, professional, or gamer you'll be able to  choose or configure an iMac to suit your specific needs.  Apple offers a range of four default configurations but you can customize an iMac to your unique needs.  The affordable 20-inch model is an ideal choice for buyers who don't need the extra size and specifications offered by the 24-inch models.  With a starting price of just $1,199 you'll be sure to find an iMac for those essential tasks such as email, web browsing, and iLife.  If you need a more powerful system, you have three 24-inch pre-configured systems above the base model to choose from that offer processor speeds up to 3.06GHz.  The higher end models  possess a larger viewing area and pack more power to run the most demanding Apple and third-party applications.  The iMac can satisfy the demands put forth by a range of users including  home users, students, gamers and most professionals.

The 24-inch model offers a more "professional" configuration and serves as an ideal choice for users who don't need a the performance and expansion options found in the  quad-core  or eight-core Mac Pro.   The iMac is the perfect middle of the road solution as it offers buyers a great combination of performance, screen size, and features.  If you use graphic intensive apps or are interested in a gaming, your iMac can come equipped with as much as 512MB graphics.  Start with one of the base configurations and simply configure an iMac to meet your needs.

What can you do with an iMac?
The all in one design of the iMac makes computing on a Mac seamless and enjoyable.  The new iMac models are more versatile than ever and allow you to:

• Accomplish daily tasks such as email, web browsing, managing photos, editing and sharing home movies, designing DVDs, designing websites, and listening to and composing music
• Truly experience Front Row with the included Apple Remote as you easily navigate through your photos, music, movies, and videos.  Front Row is interoperable with iLife and ideal for the iMac so you can sit across your room and enjoy the show.
• Use the built-in iSight camera to video chat with family and friends, make a quick video clip, or play around with the snapshots you captured in Photo Booth.
• Easily burn CDs and DVDs with the built-in SuperDrive
• Use the built-in AirPort Extreme for wireless networking
  
• Connect to the latest Bluetooth enabled peripherals and/or mobile phones
• Listen to music with the built-in speakers, your iTunes library will sound great
• Add a second display (extended desktop mode and video mirroring)
• Connect other devices such as VGA monitors and projectors
• Use Apple's full range of software from iLife to professional titles such as Final Cut Pro and Aperture

Conclusion & Buying Advice
By offering a range of iMac models, Apple assures that buyers will be able to find and configure a system to suit one's needs and budget.  The 20-inch model is the sweet spot for most users but the 24-inch model is now more afforable than ever. We recommend that you stay within budget and keep your use case in mind when you decided to buy.  No matter which iMac you choose, rest assured that the iMac will allow you to fully experience what it means to use a Mac.

Updates

• April 9, 2009 - Updated content for new models released in March 2009, updated images
  
• April 28, 2008 - Updated content for new models and placed new images
• August 28, 2007 - Updated images and content of iMac to reflect new aluminum enclosure (Mid 2007) models

Images used in this guide are Courtesy of Apple