Cryptography and Cybersecurity

• Cryptohack (capture the flag crypto challenges)
• CrypTool (educational tools)
• Cryptology ePrint Archive
• Have I been pwned (check for leaked credentials)
• Browserling (sandbox browser)
• One time secret (secure password sharing)
• KeePassXC (free password manager)
• VirusTotal (virus and phishing checker)
• MxTools (DNS recorder scanner)
• Learn DMARC (DMARC scanner)
• CSP Evaluator (Content-Security-Policy scanner)
• Merkle map (discover subdomains)
• Merkle town (Cloudflare certificate transpareny)
• Webhook generator
• Passwort (German podcast by heise)
• Reddit Sysadmin (huge subreddit)
• Brave (browser without ads or tracking)
• Proton Mail (secure email provider)
• Kali linux (distro for pentesting)
• Tails (live distro for anonymity)
• Ventoy (multiple isos on one live stick)

Literature research

• MathSciNet Reviews
• zbMATH (formerly Zentralblatt)
• Digital mathematics library
• Core - open acces papers
• The European digital mathematics library
• Elektronische Zeitschriftenbibliothek
• Math-Net.ru
• ArXiv
• archive.org
• Subito
• Göttinger Digitalisierungszentrum
• Zentrales Verzeichnis digitalisierter Drucke
• MacTutor History of Mathematics Archive
• Groups, Representations and Cohomology Preprint Archive
• Benson's list of journals
• Springer math books
• CUP Algebra books
• AMS Graduate Studies in Mathematics

Computer algebra systems

• GAP
• Sage (online interface)
• Wolfram Alpha
• Magma (online interface)

Tables and data bases

• Database of abstract Groups
• Group names
• The Group Properties Wiki
• List of simple groups
• Numbers of small groups
• Orders of non abelian simple groups
• The Modular Atlas homepage
• ATLAS of Finite Group Representations
• Atlas of Lie Groups and Representations
• The On-Line Encyclopedia of Integer Sequences
• A Database for Number Fields
• The L-functions and Modular Forms Database
• A Catalogue of Lattices
• Database of designs
• Blocks of finite groups
• List of symmetric functions
• Best known linear codes
• House of graphs

Forums, blogs, homepages

• Math stack exchange
• Math overflow
• Conference listing (FD list)
• Research seminar calendar
• Number Theory Web
• Kourovka Notebook
• Advances in group theory and applications
• Terence Tao's blog
• Pronunciation of Mathematicians' Names
• Detexify

Jobs and societies

• MathJobs
• Mathematics Jobs Wiki
• AMS Digital Mathematics Registry
• Mathematics Genealogy Project