One of these is preventing the programmer from being hired by "the client" for a given period of time after having left the firm. This period may vary from three months to two years. Some go further and prohibit working for another consulting firm.

That's actually quite a common clause. It happens outside the IT world as well, and elsewhere in the world too. There are actual understandable reasons for that - the rules aren't there to annoy. It give companies some barrier against competitors that want to hire their personnel. Or to have some leverage if an employer leaves, and goes working for a competitor, taking their customer list with them. Also note that there's a difference between having a clause in the contract, and actually enforcing it. I've signed several contracts like this in the past, but, AFAIK, such a clause has never successfully enforced in court in the jurisdiction I currently live in. It has always been thrown out as "unfair".

Many companies with software development departments require temporary resources and hire them through consulting firms. Companies theoretically avoid the expenses of recruiting and selecting, although they end up paying more to a consulting firm than if they hired the same programmers directly. What is then the advantage of hiring a consulting firm?

They'll end up paying more to the consultancy firm than if they hired the programmer directly, but they need less people themselves in their HR department, and they'll spend far less resources in sifting through candidates. Don't forget, candidates you reject cost money as well. Furthermore, (and this obviously depend on the jurisdiction), firing people cost money too. If you hire consultants, it's much easier to say you don't need them anymore starting tomorrow (because you either no longer have work for them, they aren't as good as you expect, or because they have bad BO). Once you have hired someone, even if that's for a limited time contract, it's harder to get rid of them before the contract period ends. Contractors are more costly than people you have hired, but they give you greater flexibility. Which may be worth the cost.

It is a virtually assured question in job interviews that of "Are you available to work overtime?" immediately followed by disapproving and angry looks if the answer is negative.

Happens outside of Mexico as well. In the country I currently live in, it's usually phrased differently. They'll say we are looking for people who do not have a 9-to-5 mentality, do you fit that profile?. Which is much harder to answer in your favour than a blunt are you available to work overtime?.

I witnessed the most extreme case of bizarre contract clauses last year. I was given a contract to sign, where I found a clause stating that "anything" built by "the resource", inside or outside the office, which could be considered exploitable as intelectual property, while he/she was contracted by the firm, would become the property of the company. I remember I asked the Human Resources employee who handed me the document if that was not an exaggeration. Her answer was that this was a mere formality, that what the contract said was never actually done, that it was only because someone wished to write it up like that and that I should just ignore it and sign it.

Now, that seems to be not uncommon in North America. I've had contracts like that as well, but I've always had it amended with a list of projects I was working on (or would consider working on) to be exempt.

I don't have a problem if the company claims ownership of anything I write in company time, and/or using company equipment. In my current jurisdiction, burden of proof would lie by the employer, and he wouldn't be able to successfully claim ownership of products created outside of workhours/work equipment/business knowledge.

I do recommend to never orally accept a line like "just sign it, it doesn't mean anything". If it doesn't mean anything, or if it's not supposed to be enforced, it doesn't belong in the contract.