Here's a list of SparkFun's more popular and unique shields. This isn't an exhaustive list of all Arduino shields (for that, check out shieldlist.org), but it's a good collection. They're sorted into semi-logical categories.
If you're more visually inclined, check out our ShieldStravaganza Video Series (Part 1, Part 2, and Part 3). These three exciting videos are filled to the brim with shields, shields, shields, oh...and more shields.
Prototyping (And Then Some)
Prototyping shields don't add much functionality to the Arduino, but they do help in other ways. These shields might do something as simple as breaking out the Arduino pins to screw terminals. In general they make wiring to the Arduino easier.
- ProtoShield Kit - The self-titled star of this category. This shield is basically a big prototyping area. You can stick a mini-breadboard on top, or just solder directly to the shield's prototyping area.
- ProtoScrew Shield - Like the ProtoShield, but each pin is also broken out to a screw terminal. Handy for connecting to external motors or heavy-duty sensors.
- Go-Between Shield - The intention of this shield is to sit in between two shields. It swaps the pins of the top shield, so they don't interfere with each other.
- LiPower Shield - This shield allows you to power your Arduino with a Lithium Polymer battery.
- Danger Shield - The most awesomest shield evar! This shield is a crazy conglomeration of displays, potentiometers, and other sensors. Great for learning the ins and outs of Arduino or incorporating into audio mixing projects.
- Joystick Shield Kit - This makes your Arduino a bare-bones controller. With a joystick and four buttons, this makes for a great robot controller.
- microSD Shield - The Arduino has limited storage space, but this easy-to-use shield (along with the SD library) allow for plenty of extra storage.
Ethernet, WiFi, Wireless, GPS, Etc.
- Arduino Ethernet Shield - This is one of the more classic shields. The Ethernet Shield supplies your Arduino with an ability to connect to the world wide web. There's a great library to support it as well.
- WiFly Shield - SparkFun's WiFi Shield mainstay, this shield equips your Arduino with the ability to connect to 802.11b/g wireless networks. Then it can act as either a web server, client, or both.
- Arduino Wi-Fi Shield - This is the Arduino Ethernet Shield sans wires. This shield can get your Arduino connected to a WiFi router, so it can host webpages and scour the Internet.
- Electric Imp Shield - Electric Imp is a unique WiFi module, which looks like an SD card, but it packs a powerful cloud-based WiFi controller. This is probably the least expensive WiFi-enabling Arduino shield.
- XBee Shield - XBee's won't get you connected to the Internet, but they do provide a solid, cheap means for communicating wirelessly. You could use an XBee to wirelessly trigger coffee machines, sprinklers, lights, or other household appliances.
- Cellular Shield w/ SM5100B - Turn your Arduino into a cellular phone! Send SMS text messages, or hook up a microphone and speaker and use it to replace your iPhone.
- GPS Shield - GPS isn't as complicated as you might think. With a GPS Shield, your Arduino will always know where it is.
Music and Sound
- MP3 Player Shield - Turn your Arduino into an MP3 player. Just plug in a µSD card, add some speakers, upload the example code, and you can make your very own MP3 Playing Music Box
- Music Instrument Shield - Use the MIDI protocol to turn your Arduino into a bank of musical instruments. It can make drums, piano, woodwinds, brass, and all sorts of other sound effects.
- Spectrum Shield - The Spectrum Shield listens to audio, and sorts it into bins of different frequencies. Use it to make a nifty graphic equalizer display.
- VoiceBox Shield - Give your Arduino a mechanical, robotic voice.
Displays and Cameras
- Color LCD Shield - Equip your Arduino with a unique 128x128 cellular phone color LCD.
- EL Escudo - Electroluminescent wire is awesome! Use this shield to add up to eight strands of EL wire to your project. You can finally make that Arduino-powered Tron costume.
- CMUcam - This camera module adds vision to your Arduino. You can use it to track blobs, so your robot doesn't hit any traffic cones.
- Ardumoto Motor Driver Shield - This classic motor driver shield can control two DC motors.
- Monster Moto Shield - If you need to drive beefier motors than the Ardumoto Shield can handle, this is the next step up.
- PWM Shield - Usually when you think pulse-width modulation (PWM), you might think "dimming LEDs", but PWM is also used to drive servo motors. This shield can be used to drive your crazy 12-servo hexapod.