T-Mobile is having another of their 99¢ SIM sales. Now through Oct. 21 all Prepaid SIM Activation Kits are on sale for 99¢ each. All sizes are available; mini (aka regular), micro and nano.
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 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.
The sale is good through Monday Oct 21, while supplies last. To get the 99¢ price use the code OCTSIM at checkout.