The single most rewarding thing about google? They make it so easy to bust your competition in the chops for using "shady" tactics. Everyone wants higher google rankings, some among us stoop to horrible depths to achieve them. Hidden text and links, redirects, cloaking...evil.
When I get bored of adding products to my site, or troubleshooting my defective PHP or SQL code, I go to google to bust the bad guys. Nothing is more rewarding than finding hidden text on that son-of-a-bi@#h's site with the #1 google spot for your favorite keyword. Now, I admit, I've dabbled in the dark arts myself a little with a few of my sites, but I would take the no-no code down after I saw that the googlebot had graced my site in the hopes that if anyone did report me, google would see that I had seen the error of my ways and repented. So the next time you get sick of staring at a page of your html for a missing ">", go to google, search for some of your keywords, check out the suspicious-looking rascals above you (look for keyword, after keyword, after keyword in the discription), check them out (the easiest way is to hit ctrl+a and see if any text shows up that you couldn't see before; there's other stuff to look for that involves checking their source, but the explanation of what to look for is beyond my ability to explain), when you find an evil-doer go to Google's report a bad guy link and send them to search engine oblivion. I don't know how long it takes google staff to get around to the reports, but I know they do it faster than DMOZ adds your site to the directory. I assume other engines have ways to report this stuff, but I concentrate on google.
Every now and then, I'll just pick an odd keyword that has nothing to do with any of my sites just to see what I find. The more competitive the market, the more complex the tricks. Even non-sales sites get into the act, you would be surprised how many police web sites have hidden text to boost their rankings, don't ask me why they care. The best one was when I found a Baptist Church’s site with hidden text and links. They were loading up on keywords like "God, Jesus, salvation" and even "satan, evil, and teen pregnancy". I decided not to report them because...well, I just don't want to have to explain that one should Armageddon come tomorrow.
Bottom line, it's fun, it's effective, and your competition never know what hit them.