DiskUsage - I like this first app probably more because it is like WinDirStat than because of its' own benefits. DiskUsage provides a graphical view of your storage on phone or card. Within the app you can zoom in and delete files if you wish. I like being able to keep track of what is taking up space.
ES File Explorer - This is a really nice app. It lets you see the structure of your phone/SDCard. The GUI is very nice. EStrongs puts out components that work with this app - a bookmark app, a task manager, and a security app. They are all free. ES gives you the opportunity to backup your phone to your card - very handy! There is a tab for LANs and one for FTP (also very nice and integrated well). You can customize the app easily with wallpaper, custom folder icons, etc.
All in all, I would recommend using ES File Explorer