Comments are always welcomed and appreciated. However, comments are also one of the biggest causes of spam on websites. If you do not have a proper comment moderation and protection system, your site will be flooded with links to spam, phishing and other unwanted sites.
Akismet as always does a pretty good job at tracking most of the comments. However, recently I have started seeing a new way of comment spam which uses a different tactic. With this new form of spam, spam commentators have started to use short URLs such as bit.ly and is.gd among others as their website URLs, as well as within the comments itself.
The problem with such URLs is that, it is usually a bit hard to tell where it is linking to without actually clicking on the link, and this in turn leads many users to skip checking it, specially on highly commented blogs. If that URL turns out to be spam, it would end up affecting your readers who would click on that link.
I do have a plugin idea which will detect short URLs and then try to fetch the final destination of the URL and display it in the comments administration panel, unfortunately I do not have much time to work on it, so it would be great if someone could come up with a similar plugin.
Are you too seeing this form of comment spam on your blog? Do you verify all the shortened URLs used in website field or inside the comment?
Update: I had written about a similar form of spam back in July 2009. However, it was only happening when you pulled in twitter comments into your blog. You can read more about it at Twitter Spam Spreading to Blogs