a few ideas...
turn the brightness on his monitor down. most people will attempt to change it and just start hitting the monitor like a crazy person.
pour a little milk under his desk every few days, or put powdered mi on his chair. it will pick up moisture and start stinking after a few days. that musty sour milk will get all over his clothes and he'll never figure it out that he is the stinky one in the office.
get on his computer and open word - go to the auto correct settings and start changing some words. not the super common words like the or and because they will notice them, but things like his bosses name, a project, turn random words into fun things like man-milk and baby batter or crotchfruit. you get the idea.
could always sign up his email address for gay porn or have magazine subscriptions sent to him at the office. even just things like lingerie catalogs or hightimes magzine showing up in the office mail with his name on it would make him look pretty creepy to others, and they would talk.
if your company has password lockout policies, frequently log off and try logging into his account using junk passwords. it will lock him out and he'll have to jackaround with the helpdesk a few times a day for as long as you want.
invite him to come out drinking with you guys and give him an address to a lesbo club. take a picture of him from across the street and send it to his wife.
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aside from the revenge angle, you could try some more passive things like
make a voodoo doll of him and beat the s*** out of it.
let karma roll on it's own and know that his sh*tty behavior is recognized for what it is and that most likely he is jealous or feeling threatened by you. It reflects more poorly on him than it does on you.
whatever you end up going with, post back with the results.
