pigdog: 3) How do Steam, Desura and other sites work?
- Do they check your IP address?
Certainly, they'll log it too, add cookies almost certainly and use other tracking techniques. That is, for example how regional pricing is typically applied on those sites using it, and how login details are remembered.
GOG does this too though if you're logged on and it
is standard practice, the only real question is how long they'll be retained, whether it's integrated into other strategies- for examples partnerships with other firms, use of client software- and whether they do personal rather than aggregate tracking via it.
- What else may they be able to collect?
If they install a client or their own software they
can collect whatever they want while it is running- limited by legality (which varies by jurisdiction), practicality and whatever they say in their EULA
and privacy policies (which people tend to forget about). The only way to know the policies for a specific company is to actually read the policies through. They're generally fairly standard on the privacy front, mainly varying in terms of the amount of legalese used. The typical stuff collected will be things like uptime, what games you play for how long, any sites visited, friends and connections, achievements etc but there are examples of more direct monitoring of in game behaviour as well.
Overall it's similar to the Facebook model. The 'free' client isn't really free, they'll try to sell you stuff through it at minimum and definitely use it to profile you at least so far as it helps them sell you said stuff. So it's a balance between the usefulness of the service and your attitude to privacy or any other considerations as to whether you use them.