[paul cotter (OP)]

I often see posts with links with  "no follow"  added in , by a mod, I presume. I initially thought that invalidated the link but subsequently found out that this is not the case. What is the significance/purpose of these additions?

[Zer0]

It has Something to do with Search Engine Optimization.(SEO)

If it's no follow it probably prevents Bots from utilizing them.

Perhaps a Technique to demotivate Spam by Delinking it so No search result gains can be attained.

ps - something like that, i donno, lol.

[Zer0]

Just checked...

TNS has a CAPTCHA for Registration process.

But Spammers are so so Persistent!

ps - poor MODS suffer in Silence!
: (