Dave Bergert's blog Insightful payment systems thoughts by my OLS colleague, Dave Bergert, CISSP, CISA, CompTIA Security+, and former Visa-certified QSA.
Glenbrook Partners' Blog List Glenbrook Partners has compiled "a current summary of the latest content from some of our favorite payments and banking blogs based upon their RSS feeds." Alejandro, Dave and I are on the list, as are many other good info sources.
jPOS Faced with payment systems challenges? Start here to learn more about Alejandro Revilla's jPOS project.
Randy San Nicolas' blog My OLS colleague Randy San Nicolas writes about his wealth of experience in various Issuer- and Acquirer-side endeavors in his Prepaid Enterprise blog.
soliSYSTEMS My friend Roque Solis is our go-to guy for RFID, smart cards, chip cards, integrated circuit(s) cards (ICC), HSMs, cryptographic accelerators, DES and public-key cryptography.
Specs Online - AMEX American Express (Amex) puts all its acquirer specs online for public retrieval.
Specs Online - First Data First Data Merchant Services (FDMS, aka 'FDR') puts all its acquirer specs online for public retrieval. [NOTE: FDMS' spec repository is accessible only via Internet Explorer; this link will not work with Firefox or other browsers.]