I'm on a temporary job so they don't give me any passwords to access the sites and resources I need. Instead, they tell me to move to another computer where a regular employee is and where every password is already set and saved on the browser. 

I have to be honest, I got into the router (as they are using default credentials) to get the WiFi password so I can use it on my phone and found that it had a lot to do with the activity the company does (e.g. if they were a restaurant, their password would be coffe123). With that in mind, I just wanted to see if the same pattern was used for other types of resources such as the email address, hosting accounts, etc. and yes, they were.

When registering another domain with a new account, I guessed the password by seeing my boss slowly typing on the keyboard and, again, weak as f*.

Should I tell them? I'm afraid I might get in trouble for lurking too much.

Just as a clarification: it's not a big company, we are just a few employees and none of them but me know about computers and security, so there's no way of anonimously reporting the issue or contacting a sysadmin or IT related guy.

While there is no doubt that weak passwords are an issue for your company, I would strongly advise against telling your boss about the things that you have done.

Your company decided against giving temporary workers access to sites and resources for a reason. Not only did you gain unauthorized access to the wireless LAN by guessing the password to the router, you also extended that access by probing the credentials against other resources - Resources that you were not supposed to have the password to. You then basically shoulder surfed your boss.

While there seem to be flaws in your employers policy concerning the access to company resources, and their password policies, all of these things could be considered 'hacking' by your employer and were definitely outside of your authorization.  

If I were you I would log off the WLAN and ask your employer for the password if you want to have access to it. Apart from that you should stop trying to use other peoples passwords on any access points just 'to see if the same pattern was used'. Depending on the legal system of the involved countries you can very well face legal problems for these kinds of acts.

So what should you do with the information you have?
If your employer gives you a password to a service or a resource you could point out, that e.g. that password would easily be guessable for other people. I would not mention the other password here directly though.
If your boss seems interested you could volunteer to research password best practices for the company. If they are serious about it, this would eliminate your concerns.
If there is an IT person in the company you could bring these concerns to him as he will probably understand the need for a secure password policy better.
