Interesting you mention FastStone. I have been using that also for many years . .in fact it is my default app for images, and for printing, and for scanning. I particularly like the multi-page file builder and the contact sheet generator (although I wish they would add iptc to the captions printing options).
I use Picasa as well though, mainly as I have 10,000 office and personal images to manage and it is just SO fast at searching. PLUS it can batch-edit all selected images and give you the choice to undo those edits or save them. AND it has a one=click upload to Smugmug. I have been using Picasa since the days when you had to pay for it! bit I see very little evolution in its basic abilities since google took charge.
Now and then I find I need to clone-out stuff or repair images so I was searching for a lightweight app which I could launch from Picasa on the occassions I need to clone. Thats how I found paint.net (via Paint Star and Artweaver). I still fire up photoshop now and then and get stuck into some serious image manipulation but its maybe only once a month.
I can launch paint.net from picasa with a right-click. So right now for my daily use I need FastStone, Picasa AND paint.net for my image management. It would be nice if there was a single app that would do it all so quickly. There may just be a golden ooportunity for paint.net here . . . as some of the bigger players are making a real bloatware mess of things.