At the moment T-Mobile Prepaid SIMs are free with free shipping on the T-Mobile Website.
Mini (aka regular), micro and nano SIMs are all available. I don't know how long this deal is good for but the last time T-Mobile SIMs were free it was only for a few hours.
Update 11/5: the free SIM deal has ended. SIMs are $10 again.
The SIM Activation kits contain a SIM and an activation code and can be used to activate a phone on any T-Mobile prepaid plan.
T-Mobile normally charges $10 for SIMs and activation kits.
Even before they went free today, SIMs were on sale for 99¢ until Nov. 4. So if you miss the free event T-Mobile still has a deal for you.
Even if you don't need a SIM right now it's good to have a spare handy as SIMs do go south sometimes. An active or inactive T-Mobile SIM can also be used to get the data settings menu to appear when setting up an iPhone on AT&T MVNOs.
Hat tip to HowardForums user zeppelin73 for discovering and sharing this deal.