The scoop: AirCurve, by Griffin Technology, about US$20
What it is: About the size of a small brick, the AirCurve block is made of translucent polycarbonate (Griffin's fancy word for plastic), and includes dock connectors for attaching an iPhone or iPhone 3G model. Using a "coiled waveguide" within the base, the AirCurve can magnify the sound from the iPhone's built-in speaker, delivering as much as 10 decibels of amplification.
Why it's cool: iPhone owners know that the built-in speakers don't provide much audio amplification (in other words, they stink). The AirCurve amplifies without requiring a powered speaker source, so you can get the audio effects of a speaker system. The best usage for the device would be in an office, or a bedroom if you want to use the iPhone as an alarm clock without having to buy a separate speaker system. The device succeeds in its stated mission of amplifying the iPhone's speaker without needing a powered speaker, and the price seems right for that task.
Some caveats: The amplification is probably not enough for noisier environments (you wouldn't want to use this as a speaker at a party, for example). In addition, while the device includes space to attach a charging/synchronization cable, there were no instructions on how to snake the cable through to allow for this option.
Grade: 4 stars (out of five).
The scoop: RockStar, by Belkin, about US$20
What it is: A star-shaped device, the RockStar includes five audio input jacks for headphones, as well as one input jack to connect to an iPod, iPhone or other audio device. With headphones connected to any of the other prongs on the device, the RockStar lets multiple users listen to the single iPod device. For kids (or spouses) who want to share their iPods with others, this is a cool and simple way to share the music without resorting to a speaker system.
Why it's cool: An extra audio jack cable lets you connect a second audio player to the "network", which then provides for two new functions: You can share two devices among four other headphones (alternating songs if you so desire), or you can play both devices at the same time, creating some on-the-fly "mashups" of audio content (apparently, all the cool kids are doing this). You can also use one of the headphone jacks to connect an external speaker to the system, which would let you broadcast your mashups to everyone in the room if you wanted.
Some caveats: No real negatives, other than there's not much more to the device than the headphone extension. It is what it is.
Grade: 4 stars.