iPhoto - In an opposite to the general concenus it's THE application that converted me to the Apple way of doing things. Before using iPhoto i'd just organise photos into folders on my PC manually. Not that easy to go through and view really and a bit of a chore. Maybe using Picasa or Irfanview to view them.
Now I just plug in my camera, import into iPhoto and all the work is done for me into an easily viewable library. I am an
extremely big fan of Events. I also like how iPhoto stores photos in the Original & Modified folders inside that big iPhoto Library package. That way I always have a "pure" version of my photos - I see it almost as having the digital equivalent to negatives (yes, RAW would be the actual equivalent, but you get my point

).
Picasa is also very good, but I actually
dislike it not having the option to go all controlling on my photos and I don't like how it sorts photos you've made edits to (putting the original into a hidden folder). That's down to personal preference more than anything else though.
iPhoto was the application that "liberated" my photos from a folder that would just be there to store them really to an application to store, sort, gather and most importantly view and enjoy them
iMovie - Only tried it once iMovie '08 came out and it picked up all the movies that iPhoto had imported from my digital cameras. Decided to play around with it and have had some very good results from it. Use it more because it's there instead of having some real use for it. Although it can and has come in handy on more than one occasion.
Garageband - Used it once to do a fine-edit of some audio I was putting into iMovie. Other than that I don't have a use for it, unless I start learning guitar or something.
iWeb - I don't use MobileMe so I see this as a bit pointless.
iDVD - I suppose this is still kinda useful but since sharing GB video files isn't a chore on 24Mbps broadband its relevence is slowly being eroded away. I can see why Apple don't feel a need to update it.