I was making a commenting system for my website and i was wondering if it would be a good or bad idea if i just let anyone comment by anyone i mean non members or should i make it so only members can comment?
As pritaeas has mentioned, bots can easily post comments even if registration is required.
As a cheap and easy method of implementing an anti-spam system (shall stop basic bots but anything more advanced would be able to pass through) you could include a hidden filled which if filled should be rejected. Members of the public won't be able to see this and therefore it won't be filled in however since bots often fill in any field with data regardless it can stop this type of attack.
Another method would be to implement a CAPTCHA system which requires a user to answer a question, or to repeat the words shown in an image. You need to be careful however, these are considered one of the most annoying things on the world wide web, and so logical questions might be better (such as Daniweb uses 'Are You a Human?' and 'Are You a Bot?'.