I get at least 100 attempts a day to add crap into my Guestbook. I also get a number of attempts that show up in my error log to access non-existant guestbook pages and scripts. Most of these, I imagine, are attempts to add links to their or their client's websites to improve search page ranking in which part of the page ranking algorithm is based on how many other pages link back to it. I've noticed a few of these attempts have been from other webhost companies.
I remember one day about a year or two (time flies) ago, I came home to discover 55 ads in my guestbook for Online Casino Sites. This was before I added all the protections I have now.
Stupid bots, hackers..
