Lithuanian surname dictionary can be useful in genealogical research, as it lists both the surnames known in Lithuania, and the locations where they were recorded, based on 1930s data. So, if you don't have any other information about where in Lithuania your ancestors came from, this tool may provide important clues allowing to advance your research, at least for the less common surnames.
There are two online interfaces to the dictionary data, instructions on how to use them are included in subsections below.
On the landing page, scroll down and click the green "Sutinku" ("Agree") button, to indicate agreement with conditions of use presented on that page. You can use online translation tools to translate the page before agreeing, if you are so inclined.
On the next page, type the initial letters of the surname you are interested in into the search box. The auto-completed results will be immediately displayed in the drop-down below. You don't need to worry about diacritical marks on Lithuanian letters, for example "C" will match both "C" and "Č", "S" will match both "S" and "Š", etc. You can either select the individual surname of interest from the list, or click the search button next to the box, to display all surnames matching a particular prefix.
Typical results entry contains the surname, followed by an abbreviated list of locations where it was known in the 1930s. After each location there is a number, indicating the number of families with the given surname, recorded at this location, allowing you to quickly see which locations were the most "popular" for the given surname. The number in brackets following the location list is the total number of families, a sum of individual per-location numbers. Most of the entries are followed by a sentence or two decribing the etymology of a particular surname (in many cases they are just guesses). Pointers to the surnames of similar origin may be provided as well.
In order to decode the locations, consult the location abbreviation page. To access it, click on "Sutrumpinimai" ("Abbreviations") at the top bar, then select "Gyvenamųjų vietų vardų sutrumpinimai" ("Abbreviations of the location names") from the menu. You probably want to open that page in a separate browser window while using the dictionary, for easy lookup.
Appears to be backed by the same (or, at least, very similar) data, as the dictionary website above.
You can search for either a complete surname, or construct a more complex search query, by using special symbols ? (question mark) and * (asterisk). The former matches any single character, the latter matches any number of characters. For example, search query St* will match all surnames starting with St. It is important to note that differently from the dictionary you need to use the letters with correct Lithuanian diacritical marks for the search to work correctly. If you don't have specific Lithuanian letters on your keyboard, you can enter them using the on-screen keyboard, click the keyboard icon on the right side of the search box to use it.
If the search is successful, matching surnames will be displayed on the left-side panel. Clicking on any of them will display more information. You can further click the "Išskleisti" ("Expand") link, to display the associated locations for that surname, and the corresponding family count.
A useful feature of this interface is filtering the search on location. To take advantage of it, click on "Detalioji paieška" ("Detailed search") on the front page. You can then restrict your queries to a specific location, by choosing one from the "Vietovės" ("Localities") menu - they will autocomplete to available ones as you type. Obviously, only larger locations are mentioned in the database, so if the village of your ancestors is not listed, try the nearby larger villages or towns.