Part of the Click Frenzy promotion. Just add Summit Club membership to your cart, enter the code and you're done! No purchase necessary.
They will send out a physical card to you for free if you wish. Membership provides year-round discounts as well as access to exclusive offers. At the moment as part of Click Frenzy, Summit Club members are also getting 40% off instead of the usual 20% off.
It's free, so no harm in giving it a go :)
Sale is on until 27th May, however the free membership is only on until
7pm tomorrow, 20th May, so be quick! reports in the comments that the offer is still going even though it was meant to have expired by now.