NoFollow tags tell search engines not to count links in the comments when calculating the hosting page's PageRank.
They do not prevent the search engine from following the link, and indeed, they do follow it.
This helps you in several ways:
- It gets your name out, and people who read your comment can more easily visit your site;
- It adds your link to some of the lesser-known metrics, like Google Blog Search Links and Web URL Mentions;
- It may count as a backlink, boosting your PageRank. Other factors might reduce or alter this, but you will discover, if you use WebMaster Tools, which blogs show up in backlinks. Some extremely popular blogs do.
- It allows people who are inspired by your comment to visit your site immediately.
And, of course, it takes but a second.
Note that a link within a block of text has more value than one lurking under your signature. Thus certain ways of mentioning your link have more value, depending on the type of encoded instructions on the site:
"Hi, good post. I do the comic LilNyet.com and I think your comments on coloring techniques will help me understand how black differs on programs like..."
is better than
It's an open question whether to ad the http prefix. It makes it less readable, but I understand some devices do not pick it up as a link without it, like certain phones and Twitter apps.
Please don't make comments just to post your link. If you have nothing to say, don't comment. If your site is not appropriate to the topic, it's probably better to leave it off.
If you think about it, it's almost rude not to leave your affiliation.