Magento has released a v2 of this security patch to address many of the issues from the initial patch. If you applied V1, you should revert and then apply V2.

Tool that checks if your custom checkout templates require a form key:

https://github.com/experius/Magento-1-Experius-Patch-Helper#magento

Tool that adds a form key to your custom checkout templates.

Linux: https://gist.github.com/schmengler/c42acc607901a887ef86b4daa7a0445b

OSX: https://gist.github.com/aadmathijssen/fa5f30b7e5a59a57fff3f78d1404e3a1

SUPEE-8162

Community information:

https://magento.stackexchange.com/questions/175167/supee-8167-paypal-ipn-patch-safe-to-install-right-now

PayPal post:

https://www.paypal-knowledge.com/infocenter/index?page=content&id=FAQ1916

SUPEE-9652

Community information:

https://magento.stackexchange.com/questions/158666/securitypatch-9652-possible-problems-after-supee-9652-applied

Information regarding mail sending settings in the Magento admin:

https://magento.com/security/news/new-zend-framework-1-security-vulnerability

SUPEE-8788

Community information:

https://magento.stackexchange.com/questions/140550/security-patch-supee-8788-possible-problems

SUPEE-7405

Community information:

https://magento.stackexchange.com/questions/98220/security-patch-supee-7405-possible-problems

SUPEE-6788

Community information:

https://magento.stackexchange.com/questions/98220/security-patch-supee-7405-possible-problems

Tool to check Magento files for appsec affected code:

https://github.com/Schrank/magento-appsec-file-check

SUPEE-6482

Community information:

https://magento.stackexchange.com/questions/76600/magento-security-patch-supee-6482-what-is-patched

SUPEE-6285

Community information:

https://magento.stackexchange.com/questions/73481/supee-6285-patch-what-has-been-changed