Have something you want to promote? Are your friends getting tired of the spam you post in your journal? Got kicked off of a community because you strayed off-topic to promote your own stuff?
Then come here, a whole community dedicated to plugging... well, whatever you want, really.
Your own LJ community. Your art gallery, or web comic. The fic you wrote. Your Neopets guild or shop. Your sex site, even. Or go a little more commercial, and advertise your online store, your currently running eBay auctions, or your MLM or GPTR program.
Anything goes... although it would be nice if you put up a decent warning if you're really promoting something that goes beyond a PG rating, as you never know who might stumble upon your ad. ^_^