A great resource for finding information on German emigrants is the Deutsche Auswanderer-Datenbank (German Emigrants Database) at the Historisches Museum Bremerhaven. This project collects information on emigration to the United States. The main focus of the database is on European emigrants who left from German ports.
The German Emigrants Database collects passenger manifests principally between 1820-1897 and 1904-1907. The site also studies European emigration to the US and collects information about German emigrant ships.
You can search online for the names of emigrants in their collections or the Friends of the Museum Association will also perform searches for you for a fee.