Direct 5V current based on analog signal
If you have 4 pins available, you could use charlieplexing to selectively light up 12 led.


Alternatively you could use some IC. You could use the one Greg suggested. 

Or you could use a shift-register. This requires only 2 or 3 pins, an will enable you to light up any one or more leds at the same time. Most have 8 outputs, but you can connect one to the other (daisychain), so with 2 shift-registers you can individually turn on/off 16 leds. You can easily control these using arduino's shiftOut command.

You could also use a port-expander using I2C to talk to it. More complicated, and a bit more expansive. But also more powerful. It will require only 2 pins (A4, and A5).