A very simple gallery I've written in PHP - only requires one file, and has no database backend. The only absolute requirement is a Linux server and PHP.
If you have mod_rewrite installed:
<IfModule mod_rewrite.c> RewriteEngine on RewriteRule ^(.*)/$ index.php?path=$1 [QSA] </IfModule>Browse to that folder, and you should be up and running.
If you do not have mod_write installed:
Upload files and directories via FTP into that folder; Gallery will take care of the rest.
Thumbnails are automatically generated for JPEGs, GIFs, PNGs and BMPs, and placed in the thumbnails folder.