RESUMEN: Complex systems like a cell, the financial market or a society, exhibit non-trivial behavior. Often, empirical data obey a scale free distribution in the frequency of observations or more specifically Zipf's law that states that the size of the k the most frequent observation should be proportional to 1/k. Mora and Bialek translated this observation in statistical mechanics terms, by observing that systems that exhibit this behavior are akin to critical systems, i.e. systems close to a phase transition in physics. Since this is a very special point, this raises the issue of what universal mechanism may be responsible for the self-organization to the critical point. After a brief (and biased) review of the problem, we discuss these issues in a simple framework inspired by maximum entropy considerations.