Is there a plugin that logs my usage of plugins on the blog? (when I downloaded a new plugin. When I activated/deactivated it and so on) ?
I have searched fairly extensively, and have not found such a plugin. The best recommendation I could make is to use the WordPress File Monitor plugin. This will at least let you know when a new plugin is installed. Really, you should be using it regardless - it's a great plugin to use as part of an overall security strategy.