How to run 25 RGB LEDs by an arduino Duemilanove
I can see two ways of achieving this.

1-Using a technique called Charlieplexing to control your leds.
With 10 output pins you would be able to control 90 normal leds. The main disadvantage with this technique is that the leds are not always on. They flash really fast, causing a reduction in the brightness of the leds. It's also a bit complicated to code and I'm not 100% certain that you could make it work on RGB leds.

2-Using Shift registers. Like this one.
With shift registers you can ouput data in a serial fashion and output it in parallel. Depending on the number of pins you are willing to use, you'll have to buy more or less Shift registers. The main idea is something like: output the new states of the leds, one by one (separating your signals by outputting HIGH to the SRCLK) and then "applying changes" by outputing HIGH to RCLK. If you think about it, you can basically cascade to an infinite number of outputs. You'll need a lot of shift registers though and the response time will probably be slow.