Visual inspiration apps are becoming more and more common. Pinterest, Icebergs, Ember and many more are great services that offer people to collect and store content - mainly images. Icebergs is personally my favourite because it doesn't just store images, you can also store a variety of other content including notes, bookmarks, files and much more.
However if you're looking for something that simply sits on your computer, stores all the content on your computer and is free to use, Inboard is a great option.
Inboard is a visual inspiration app that is aimed at designers, but can of course be used by anyone. It of course supports images and can also take PDFs, PSDs and other similar files. If you import a PSD, it'll even show a preview of it so you know what it is before opening it up in Photoshop.
You can simply drag and drop content from anywhere on your computer. A great feature included is the ability to drag and drop content from the web, it'll retain the source link incase you ever want to go back to where you got it from in the future.
You can apply multiple tags to content for easy organisation and search. Double clicking on an item enlarges it and gives you more information about it.
You can also organise content into different folders - great for if you're working on projects or just categorising your content more specifically.
Inboard is currently free and is in BETA. So there will definitely be some bugs. If you are going to download the BETA, help the developers by providing any feedback.