ssl and certs
Certificates expire at the worst possible moment, and openssl commands are hard to remember. These two help with both.
checkcert.net
Monitors your SSL/TLS certificates and emails you before they expire. It checks them on a regular schedule over plain TLS, SMTP STARTTLS, IMAP STARTTLS and more, and shows issuers, validity dates and certificate chains in one dashboard.
openssl howto
The openssl commands I keep looking up: creating keys and CSRs, self-signed certificates, inspecting certificates, fingerprints,
PKCS#12 conversion and testing servers with s_client. It also has a CSR checker that decodes a pasted CSR in your browser.