I have had the privilege of publishing in a number of electronic journals related to number theory and combinatorics. Some examples include:

One of the most impressive sites I have ever
seen
dealing with the history of mathematics is housed in St. Andrews,
Scotland
and is known as the MacTutor History
of Mathematics Archive.

Neil
Sloane's
Online Encyclopedia of Integer Sequences

Ed Sandifer's "How
Euler Did It" Column for MAA Online

CTAN, (The
Comprehensive TeX Archive Network) is the home of the files needed to
implement the TeX typesetting language

A list of websites
for professional advising in mathematics that I compiled some time
ago