According to the wiki, the GPIOs are 3.3V
Broadcom seems to be somewhat stingy with their product info, but i wouldn't source more than 20mA from any pin, just to be safe, i would try to be lower.
I didn't find any info on whether the pins are 5V tolerant, so i wouldn't risk it.
LEDs are a wide range, i usually source mine with 2-350mA, so the answer is maybe
If you just want an indicator light, many modern LEDs do pretty well at 5mA, that should work directly. But you are probably going to have a problem driving blue and white LEDs from 3V3.