If you're offering a free resource, then most savvy user's generally think to themselves "what's the catch?". So it's not a great surprise that with these free resources come with a caveat of signing up to a mailing list or divulging some information, usually your email address.
What you need to do is create a balance between asking for not too much in exchange for this freebie. Ask for too much or in the wrong way and people will be put off and not bother, ask for too little or for useless information and you haven't made the most of offering this freebie.
It all comes down to the reason you are offering this freebie. If you want to simply offer something, increase your brand image, look generous, then you don't need to ask for anything in return. If it is purely to increase sign ups to your mailing list then think how much info you need from them.
Maybe all you need to do then is to ask for their email address. If this is the bare minimum of info you need to sign them up. Also, rather than a download, send the freebie to their email address so they are forced to enter their real address in and not a fake one.
Keep the process simple, open and honest. User's will respect you asking for an email address to send the freebie to and understand that in return they will be subscribed to your mailing list (if you make it clear to them). Chances are if they are interested in a resource you have offered then they will find it useful to be on your mailing list for future content too.
For example (from uxpin)