SimpleK12 is a teacher community; it is free to join as a basic member and there is also a full membership, giving lots more for anyone who wants to pay. Teachers can try it out and see whether it is useful for them before parting with any money.
A very interesting feature of this site is that they do regular webinars on all subjects. These are free for basic (free) members to sign up for. You do not get a certificate for attending, but for the UK this is not a real issue – certificates are designed for countries where teachers have to prove so many hours of CPD external to school.
SimpleK12 provides many resources free on demand – I searched for the topic light and received a small list from which I chose a webinar about digital ink.
From the image you can see what it was, a thirty minute webinar with free access, that I could watch when I was ready. (NB I am pretty sure that some webinars are not for free access)
There are also many free ebooks. Some of these, again, are paid for. If you see one that says VIEW to the right (see image example below) then it is free to download and share. Otherwise there is a price shown instead of VIEW.
You can download the free e-book about iPad apps (illustrated above) at the link below:
All in all, there is a lot for teachers on this site and it is well worth having a look at it.