Quick, feature rich and easy to use multi-platform image viewer XnView  replaced on my PC sluggish, awkward and non free ACDSee a long time ago. Firstly I use XnView just for viewing and cataloguing images but soon I was amazed how much wonderful functions XnView has. In addition, XnView supports a lot of image file formats (read more than 400, write 75) has customizable interface and layout in Browser mode and much more useful features not mentioned in this list.

1. Resize images
XnView let you choose Lanczos from resize algorithms. Since Lanczos is much more precise than Bicubic (which is used by default in Photoshop) it is the best way to resize images without quality loss. (Shift+S)

2. Optimize jpeg files
Believe me or not but XnView compress jpeg files much more effective not only than Photoshop but any other utility I’ve ever seen (Ctrl+Shift+S, click options and check “optimize Huffman table” and “rebuild embedded thumbnail”)

3. Lossles image rotation and flip
Not every tool provides such easy and efficient way of rotating images lossless but XnView does (Shift+L rotate left, Shift+К rotate right).

4. Display color information and count colors used
There is no need to open Photoshop every time when you just want to know exactly RGB and HTML color values (Ctrl+Shift+I).

5. Quick image improving and feature rich edit
Besides Automatic Levels (Ctrl+Alt+L), Automatic Contrast (Ctrl+Alt+C) and Sharpen (Filter->Enhance->Details, Edges, Focus) those I use very often working with images for web, there are also more adjusting tools and filters like Levels, Hue/Saturation, Color Balance, Remove red eye, Swap channels, Add text etc.

6. Set image as wallpaper
There are five options to set image right from XnView as your desktop wallpaper or return previous one (Tools ->Set as wallpaper).

7. Remove all metadata and edit IPTC data
Protect your privacy by removing all metadata from your photos for web sites and easy edit IPTC data without installing any additional tools (Edit->Metadata). It’s a pity, there is no batch edit of IPTC and no EXIF data edit (just edit of timestamp) implemented yet.

8. Batch image conversion
You can change format for all files in chosen folder, rename them, resize and apply auto levels and other edits in batch process (Ctrl+U).

9. Compress to Zip, burn with Nero, publish online to ImageShack.us and send by email
All these things you can easy do from XnView (Filter->).

10. Comprehensive image search and find duplicates
You do not need an extra tool to find duplicates of images by file size, name and even content (Tools->Find similar files in Browser mode). Search is highly customizable; besides date and Size you can choose such fields as EXIF, IPTC, Width/Height, Format etc. (Ctrl+F)