RSSJimmie P. Rodgers brought a whole bunch of color changing LED throwies to Providence recently, and I just had to get some of the LEDs he used for my own enjoyment.
Each piece has three LEDs inside that are under the control of an embedded microcontroller. There are two main variants: fast flashing, and slow flashing. The slow flashing models have more of a gradual change to them as shown in the video above. You can also get diffused and clear versions of either. Unlike the four-lead common cathode or anode RGB LEDs, there is no way to control the mix of colors (you just have to sit back and enjoy them).
The best place I've found to buy these is eBay, where Jimmie and I have both found them for around 25 cents each (shipped) to the USA. A search for rgb led slow or rgb led fast should turn up some.
It may take a couple weeks for them to arrive, but they are well worth it; use them to make some LED throwies, and let the fun begin.
I have a bunch of them - I got them from SuperBright LEDs. According to them, you don't need resistors, they're built in. And if you hook a bunch up together, they're not quite synced, so they change color in unison for a while, then it gets chaotic - very pretty!
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...'course you have to buy 100 of them, but that's including shipping from HK:
http://cgi.ebay.com/100-5mm-Colorful-Slow-Flash-RGB-LED-FREE-RESISTOR-SCF5_W0QQitemZ290266333950QQcmdZViewItem?hash=item290266333950&_trkparms=72%3A1205|39%3A1|66%3A2|65%3A12|240%3A1318&_trksid=p3286.c0.m14
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