ironspider LEDs are so popular now that there is a wide variety of LED drivers to choose from. The easiest way to drive a bunch of LEDs is to get a driver with a PWM input, for example there are several models of BuckPuck and Meanwell drivers that have it. You just connect a PWM pin on the Arduino to the driver and dust off your hands.
Usually when driving a bunch of LEDs, you want to run at a higher voltage so you can run long strings of series-wired LEDs. 12V (for 3 LEDs per string) or 24V (for 6 LEDs per string) are popular choices. Generally you'll need one LED driver per string, because (except in rare circumstances) you should NOT wire LEDs in parallel.
What's your application? If your purpose is illumination rather than display of information, consider power LEDs. You can get the same amount of light from three power LEDs as you can from 100 little ones. The power LEDs will be cheaper to begin with (per lumen) and also easier to hook up (a single 12V supply, a single driver, and a dozen connections instead of 200+). Power LEDs also generally have more/better choices of color temperature. Only downside is power LEDs will require cooling via heatsink or fan.