Sheets
Sheets (opens in a new tab) is an online contact sheet generation tool.
Upload a folder of images, and then you will have contact sheets generated and download to your computer as JPGs. Even if you don't have an internet connection, you can use an offline version of the site to create your sheets!
Note: since all of the generation is done client-side, sheet generation can sometimes take a while especially if you are trying to process hundreds of images that are very high resolution. Consider creating a low res copy of your images first before processing them through Sheets.