You could promote something in Freebies area, some forums have that area and it’s great. Just to offer something free…
Okay, quick example for those whinging about forum promotion & how they are not getting any links/work from it.
I am a member of a paid private forum, I answered simple questions about SEO, one of the people on the forum liked me, and has now outsourced his SEO to me for £XXX/month.
Had no promotion on my signature links, instead a home page link on my profile.
I didn’t do this by being spammy, I did this by being good, honest & helpful
If a forum allows posting signatures, then go for it. Otherwise it is spam. Pure and simple.
It always important to abide by any forum’s rules so as to make life easier for everyone.