GOOD PERSON ALWAYS DO JOBS ACCORDING TO THE RULES.

This is the only place where you're allowed to promote your free services.

There are no definite limitation here as long as you don't break these rules:

This is the only place where you're allowed to promote commercial based sites.

They're not definite limitation here as long as you don't break these rules:

Don't promote any service connected to s*x or brute.

Don't promote any service that cause hatred to any organization, religion or country.

Don't promote any service that will harm people or animal.