But I wonder if this is a good thing, and whether we need some sort of policy to prevent the forum becoming cluttered with referral links? After all, it does mean that you have to read these same links time after time when reading posts by a particular member. And surely that value to other members (of informing them of other sites of interest) could be achieved just as well by having a discussion topic about such sites?
As a 'mutual' company, it's not for me to impose any rules. It's up to you lot!

** UPDATE **
Polls on specific signature rules
Charge for signatures? RESULT: 14 in favour, 20 against
Restrict links in signatures to "imutual" ones? RESULT: 8 in favour, 4 against
Allow banners / images in signatures? RESULT: 1 in favour, 10 against
Restrict font size in signatures? RESULT: 14 in favour, 1 against