@Amigamancer
If you're into astronomy then Digital Universe v1.5 comes highly recommended (
http://www.digitaluniverse.org.uk/)
Also on my list of OS4-only SW, as others also have mentioned, are:
DVPlayer - Best video player ever on the Amiga!
Epistula - Multiprotocol messenger client. Still lacking some features but getting there.
SabreMSN - Same as above but focusing only on MSN.
Amiphoto - I use this to manage and view pics taken with my digital camera.
AmiDVD - Great for burning files to a DVD.
TuneNet - Music player with a *lot* of plugins + Internet radio support (shoutcast style)
Cygnix X11 - Opening a new world of apps from Unix/Linux.
WW2 - An Axis & Allies clone. Great potensial, and I rule when I get to play Germany.
Wet - Everybody talks about the weather. Now you can watch it on your Amiga.
Transmission - Fast and easy to use torrent downloader. Has some problems with SFS partitions though.
CPUTemp Docky - A nice little docky that shows CPU usage, CPU temperature and as a bonus; the time.
Apologies to the creators of everything else I've forgotten...
edit: Ah, how could I forget TFB?! - Hours of fun for both kids and adults!
Edited by Monoxyde on 2008/5/10 18:19:22