ArXiv Front: NT Number Theory: Very large Number Theory section of the mathematics e-print arXiv

Computing Minimal Equal Sums Of Like Powers: Jean-Charles Meyrignac's distributed-computing project on equal sums of like powers and the place to look for the current status of the problem

Equal Sums of Like Powers: Chen Shuwen's extensive collection of equal sums of like powers, especially those that are solutions to multigrade equations

Great Internet Mersenne Prime Search (GIMPS): Large distributed project to primality test all Mersenne numbers

Number Theorists' Home Pages: Over a thousand home pages collected in the Number Theory Web

Number Theory Conferences, New and Old: Compiled by Keith Matthews as part of the Number Theory Web

PARI/GP Development Headquarter: Main site, with downloads, documentations, CVS, mailing lists, and FAQs

Primality Proving: Covers different types of primality tests, such as quick, classical and general purpose prime filters. Page includes bibliography

The Prime Pages: Prime number research, records and resources. Includes largest known primes, finding and proving primes, glossary, curios, links

The Prime Puzzles & Problems Connection: Carlos Rivera's collection of problems and puzzles related to prime numbers