[SIZE="7"]DON'T DO THEM!!![/SIZE]
Signature advertisements are strictly forbidden in the forums. The only exception is personal sites (e.g. a blog you own or a non-commercial website you created (sans free-to-play games)). Signature advertisments will be immediately removed by staff and may even result in a permaban if you are suspected of being an advertising chatbot (a.k.a. adbot).
If you are unsure whether the site you wish to link to in your signature falls under the advertisement exclusion rule or not, please PM a staff member to get their ruling on it. Do not place the link in the signature anyway.
This thread comes from the virulent use of signature links and advertisements by new members. If you are going to join the forums only to put a site link in your signature, then don't bother joining unless you fall under the personal site exception.
Finally, to punctuate our advertisement policy:
FAQ No advertisement threads or posts DO NOT create advertisement threads or posts for off-site things such as any type of referral site or trading/selling DVDs, games, clubs, websites, contests, etc. (NOTE: You may advertise your personal site in your signature.) DO NOT send unsolicited private messages to our members advertising things such as (but not limited to) any type of referral site or trading/selling DVDs, off-site clubs, contests, etc.