JewishGen.org is the premier site for doing Jewish genealogical research. It is also the host for this website. The site contains very much information, so take some time to explore it. Some of the most useful parts require registration, but it is free. Some of what it includes:
The Jewish Genealogy Family Finder (JGFF): You can list family names and names of towns that you are interested in. If someone is interested in the same family and/or town, they can contact you. Similarly, you can search the database to see if anyone else is researching the family and town you are interested in. It does work!
Family tree of the Jewish People (FTJP): Family trees that people have submitted.
Special Interest Groups: German-Jewish, Litvak, Belarus, Ukrainian, and many more.
Lots and lots of databases.
Torah scrolls. Temple Beth El, Richmond Avenue. Courtesy, University Archives, State University of New York at Buffalo