2010 Mathematics Subject Classification
Koninklijk Wiskundig Genootschap
Platform Wiskunde Nederland
American Mathematical Society
International Mathematical Union
Internet Mathematics Library
Mathematical gateways to the www
Wikipedia encyclopedia: Mathematics
On-line Encyclopedia of Integer Sequences
The abc conjecture home page
ABC triples
L-functions and modular forms database
Cremona's elliptic curve database
MacTutor History of Mathematics archive
Indian mathematics education website
Terence Tao's blog
Polymath blog

Number Theory Web
International Association for Cryptologic Research

Preprint archives

arXiv e-Print archive
Recent number theory submissions to Math ArXiv
Recent algebraic geometry submissions to Math ArXiv
Cryptology e-print archive

MathSciNet (for subscribers only)
Zentralblatt (for subscribers only)
Google scholar
Google
Website with search engines (www.whitelines.net)

Seminars and workshops

MFO Oberwolfach
CIRM Luminy
BIRS Banff
Intercity Number Theory Seminar Netherlands
General Colloquium/Stafcolloquium Leiden

Other sites

Wikipedia encyclopedia
NWO
KNAW
DB Trainplanner
APTA Public transportation links
Fahrplan online (German)
Airtickets page (Dutch)
IBAN calculator
NOS
Telephone directory
World clock