This is bugging me...

[Thinkofanumber]

Apologies if this has been mentioned before.

The 'Join Mumsnet for free' banner which is on screen when you aren't logged in.

I know it's common parlance, but it grates. Surely it's 'Join Mumsnet free' or 'Join Mumsnet for nothing'.

Just needed to get that off my chest

I disagree.
I think that your alternatives are either difficult to parse or potentially ambiguous and I think clarity trumps pedantry. :)

"For free" avoids certain ambiguities because "free" also means "at large", "liberated", etc. as well as "without cost".