Well I have done some tests and found that I can get over 250mA from an output pin of an Arduino.
Of course this will / has damaged my Arduino but it is still actually working. I did this to to investigate what sort of currents are being drawn by connecting an LED direct without a resistor. This followed some recent discussions on this forum.
I have added an LED page to my Tutorials:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html
Grumpy Mike,
You seem very far from grumpy in your generous sharing of extensive knowledge.
Your site has lots of good explanatory info on it -- an RSS feed for newly added items would be great, in case one misses a thread here about it.
Thanks lots for all of it.
.andy
That made me realize that I should probably use a resistor with an opto-isolator as well, right? Somehow I missed that before, even I knew that I should use resistors with LEDs and the other half an opto-isolator is a LED.
For some reason the circuit and Arduino Nano still seem to work after few months, but I don't want to push my luck any futher.
I checked the opto's (sharp pc817) datasheet. I am not sure if I got the terms right, but the parameters I used to calculate the resistor value was the typical forward voltage 1.2V @ 20mA (max rating for the forward current is 50mA). That led to resistor value (5V-1.2V)/0.02A = 190ohm. 220ohm is closest I have. Does that sound reasonable?
Btw, this might be a silly question but here it comes anyhow: It doesn't matter which side (anode or cathode) of the LED the resistor is, does it?
Like Andy Davidson said, thanks for all the info you share on here. I've learned lots from your posts alone!
oh and this is the best quote imo
"Does my Arduino still work after this? Yes it does! Have I permanently damaged it? You bet I have!"
lol
It doesn't matter which side (anode or cathode) of the LED the resistor is, does it?
No problem the electrons don't care what they see first.
= 190ohm. 220ohm is closest I have. Does that sound reasonable?
Yes it does, spot on. 
However, have you not got a 180R resistor, that is a standard value? Anyway it doesn't matter.
RSS feed for newly added items would be great
Good idea but I am not sure if my ISP supports it, I will have to check.