Khan Academy
Khan Academy: The Law of Segregation
Resource provides a discussion of Mendel's law of segregation with a focus on genotype, phenotype, alleles, heterozygous/homozygous, and 2 x 2 Punnett squares.
Other
Biology Project: The Gene Machine
Site allows users to learn interesting information about their genes based on his/her physical traits. Provides a brief explanation of how a person's genotype is determined.
Other
Evolution: The Theory of Natural Selection (Part 1)
These pages are part of a site called Evolution that accompany a textbook by the same name. Mark Ridley is the author. This section has an advanced discussion of determining allele and genotype frequencies using the Hardy-Weinberg ratio.
University of Maryland
Univ. Of Maryland Baltimore Cty/biology Century
Historical perspective on Mendel's contribution. Spends a little time setting up what the current beliefs of Mendel's day were and how he changed things with what he did.