T-Mobile Website through 2/13. The regular price is $10.
The SIM kits contain a SIM and an activation code and can be used to activate a phone on any T-Mobile prepaid plan. The kits are available in mini (aka regular), micro and nano sizes.
By themselves, without the activation code, these SIMs can also be used to activate a supported tablet on any T-Mobile mobile broadband plan including the 200 MB/month free plan.
I like to keep a spare SIM or two on hand to give to international visitors or to activate the 5 GB of data for $30 plan, which is only available with new activations, temporarily when traveling. 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.
Related post: Prepaid Operator Profile: T-Mobile