Ancestry is hands-down the best genealogy resource on the web. Ancestry has the largest collection of digitized documents and the best searching and matching algorithms (but you’ll still have to use your brain a bit if you want to build a solid tree). You can create an online tree and access basic documents for free, and, if you want to research further, you can set up a subscription to find original documents from around the world. Ancestry also offers DNA testing that can connect you to previously-unknown relatives.
FamilySearch is a free-to-use website with a large database of searchable original records. Additionally, FamilySearch holds the catalog for the Family History Library—the largest genealogy library in the world, with thousands of books and millions of microfilms in its collection covering sources from almost every corner of the earth.
The United States Customs and Immigration Service (USCIS) currently holds U.S. naturalization paperwork dating back to the reforms of 1906. After you have gathered solid identifying information about your ancestor, you can order an index search from the USCIS, and, if they find an entry in their index, you can then order the full paperwork of your ancestor. Post-1906 naturalization paperwork will have much more detailed information about your ancestor’s place of origin, and may also include photographs, copies of vital records, and other details that can’t easily be obtained elsewhere.
MeyersGaz is a great, free website for finding places in the pre-1918 German Empire. The creators of this website have digitzed, abstracted, and indexed the vital Meyers Orts- und Verkehrs-Lexikon des Deutschen Reichs—a gazetteer (list of locations) published in the German Empire in 1912/1913. The site then pairs this information with historical and modern maps, as well as information about local religious and civil jurisdictions.
Archion is a pay-as-you-go website, but it should be your first stop when looking for German records of your Evangelical (Lutheran and/or Reformed) ancestors. Archion is a joint effort by several Evangelical archives in Germany to digitize their historical church books. Individual records are not indexed on this site, and the church books are organized by parish and archives, so it is important to know exactly where your ancestor originated and to which jurisdiction your ancestor’s hometown belonged in order to find them in these records.
Matricula Online is a free online church book database sourced by Roman Catholic archives and primarily covers the archives in Austria, although more and more Catholic archives in Germany and beyond have been joining recently. Nearly every Austrian state is represented here, except for Burgenland, which was part of the Kingdom of Hungary until 1918, so its historical records are held in Hungarian Archives. Although Matricula Online is free, saving images is prohibited. Individual records are not indexed on this site, and the church books are organized by parish and archives, so it is important to know exactly where your ancestor originated and to which jurisdiction your ancestor’s hometown belonged in order to find them in these records.
Mapire is a free map database focusing on the collections of archives of the former Austro-Hungarian Empire, but these maps cover most of Europe and even beyond. Mapire has stitched many of these maps over modern maps and allows you to toggle between these maps to more easily find modern locations in historical maps, and visa versa. One of the more interesting maps in their collection is their composite cadastral map, which allows you to find the exact plot of land and/or dwelling associated with your ancestor.
The Register of Swiss Surnames is a little-known but vital resource for those researching their Swiss ancestors. Swiss citizenship laws meant that the Swiss government has records of citizenship dating back before the 1700s, and they have compiled a database of surnames found in these rolls and, even better, the exact municipalities where these surnames have historically been registered. Especially with rarer surnames, this database can help you narrow-down a handful of specific locations to search for your ancestor.
The David Rumsey Map Collection is a vast collection of historical maps of all shapes and forms from just about all areas of the world. My most-used map in this collection is the composite of the 1893 German survey maps, which has been stitched over GoogleMaps to allow for easier comparison between the historical map and modern map.