I had an interesting discussion with Karl Guttag from Syndiant, about the difference types pico-projectors, I thought it was a good idea to turn it into a post (thanks Karl!).

There are three main types of pico-projectors:

• Stand-alone: These are devices that are essentially just a projector. They receive the input via a cable (A/V, USB, etc) and cannot display anything unless you use another device to stream the video signal. Sometimes these include speakers, too. Examples include the 3M MPro-120, Optoma's PK-100 and PK-101 and Microvision's upcoming laser-based Show WX.
• Media-player: These projectors also include on-board memory (or a memory-card slot) and can play files directly from the memory. Obviously the projector needs to support the file type you want to view, whether it is photos, videos or audio files. Examples include the AAXA P2 and the Optoma PK-102. Some of these projectors actually include a mini PC. The Allcam CP1 actually runs Win-CE, and thus can show Office documents, includes a touchpad controller and can be hooked to external mouse and keyboard. It's a bit bigger and heavier than the average pico-projector, though.
• Embedded projector: in this case the light-engine is added to an existing product (projector-phone, camera, laptop, PDA, etc.) as an accessory. Examples include the Samsung Show phone and Nikon's Coolpix s1000pj camera.

The huge growth that is forecasted for pico-projectors lies mostly in embedded devices. When the light engines become small, efficient and cheap enough, we can expect many phones, cameras and laptops to embed one inside.

Stand-alone projectors is a type of device that some people expect to disappear in a few years. You have to carry two devices, the cables are a bit annoying (and it is harder to place the projector in a good position with a cable) - and also the price difference to include a media player is not so high. Personally, I can say that indeed it is very annoying not to have a media player built in. Especially if you just want to show-off your new gadget...

One class of stand-alone devices might still be interesting - tiny USB-powered projectors that draw both the power and A/V signal from the USB. One example of such a device is the Sanwa PRJ-001 (it's tiny, weighing only 85 grams!).