The only ones I use right now are iPhoto and GarageBand.
I tried the Aperture demo but meh, it's not for me. Not interested in Lightroom. Bonzo likes it simple and lean - for general .jpg work I'll import into iPhoto from my Nikon D50 or Panasonic FZ20, then export a copy for adjusting with Photoshop Elements. For Nikon RAW processing I copy the files from my Nikon into a folder on my HD, then work on them with Nikon Capture NX (which I like just fine), and export the developed files as .jpgs with maybe a bit of final PSE retouching after that.
I might try iWeb this Summer when/if I finally get a .Mac account. Haven't touched any of the other apps but who knows, maybe one day.