How can I organize movies, with IMDB import and fields for tags?

I've not used it myself, but heard good things about Ant Movie Catalog (yes, it's a silly title). It seems to satisfy all your demands thought - free & open source.

(more screenshots at the page linked)

The solution I use is put all the movie files into a single folder, and point Kodi (formely XBMC) to that folder. By default, Kodi does a pretty good job of getting movie meta-data just from having the title in the filename.

MBase allows you to manage your movies in an easy and comfortable way. With the help of an online database you just need to enter the title and MBase will fill out the fields with the appropriate data on its own. Covers and also even the actors (their birthdays and photos too) are automatically added.

• Free of charge with no Spyware
• Data for movies and actors off the Internet
• Loan Management to keep track of lent movies
• Statistics to analyze collection
• Wikipedia directly integrated
• VirtualFlip to browse through collection

MBase is freeware and portable.