What should I use for guest identifier?

I'm opening up a feature on my site that will not require a user to create a profile. I need to create and 开发者_开发知识库track user ids so I'm curious what others use as an indentifier for guest accounts. Here are some of my thoughts:

1. Capture the client IP address and use that
2. Assign a unique ID

Just curious if anyone else has done the same and what they used.

Use cookies. But, anyway, it's exploitable.

Don't use IP, as the IP can change very often. And, in cookies, assing an unique ID, as you marked in your question.

These days user don`t like to register over and over again. More and more of the data over internet is going to the cloud. Rob W suggests openid some info about the openids on wikipedia. Why not use facebook, google accaunt. There are other solutions - more or less what you need - gravatar, disqus these are more "comment oriented".

is regular php session inefficient? you can add to it anything just to complicate the structure, but anyway it'll be always exploitable, so I would keep it as simple as possible - use session ;)