In this post I was somewhat taken aback by seeing someone's username expressed as </a></b></a>seidoo_ryuu. I thought it was some paid account thing I hadn't heard of before, but it's actually something generated by the person making the post, and has nothing to do with the user's account.
The generator is available on this page. I wouldn't be surprised if this started to catch on in popularity, and people started putting code for their preferred image in their userinfo page. (I babble a bit more about it on my own journal in this post.