Number Theory Web   http://www.numbertheory.org/

History Topics: Numbers and Number Theory Index   http://www.gap-system.org/~history/Indexes/Number_Theory.html

Number Theory Foundation   http://www.math.uiuc.edu/ntf/

Discovering Number Theory   http://www.math.mtu.edu/mathlab/COURSES/holt/dnt/

Research on Number Theory   http://mtl.uta.fi/number-theory/

Number Theory at the Mathematics Dept. of the University of Texas   http://www.ma.utexas.edu/users/voloch/numberthy.html