Other
Aish.com: Counting the Omer With Aish
Provides links to four articles dealing with the counting of the Omer and Lag B'Omer.
University of Calgary
Sources: The Tragic History of the "Omer" Season
This resource provides information behind the Omer. Also traces the observance of the ritual among Ashkenazic Jews.
Other
Jewish Path: Notes on Counting the Omer
Many questions and answers about the Omer. Describes such things as when Jews should begin counting the Omer and what to do if the counting is skipped one night.
Judaism 101
Judaism 101: The Counting of the Omer
Provides historical background. Also explains why it is that Jews count the Omer. Provides the dates of Lag B'Omer.
Other
Torah.org: Sefiras Ha Omer What's an Omer
Provides detailed historical background about the omer.