What's with the explosion of Poly Bridge giveaways?

You cannot buy that one anymore, so "was". However, you might still be able to find a cheap copy around on steamtrades and such.

Humble monthly is a subscription based bundle received every month, so it's only available for those that paid for it before it was released.

It sells for very low on Group Buys, I'm sure you'll be able to snatch a copy. Literally hundreds of people trying to sell their copy in a non-profit manner.

Edit: ninja'd.