jivetalk is great for IM.
google calendar is designed for iphone, but essentially just gives you an agenda view.
bejeweled is now official (i guess they hired the diamante guy and cleaned it up a bit)
http://www.tictacwhoa.com is always great :-)
appmarks is a nice interface to favorites
I *love* pickleview for monitoring baseball games (much better than my treo 650 solution of constantly reloading wap.mlb.com)
There is a nice version of minesweeper available.
I'm about to release Vegas Solitaire (http://www.tictacwhoa.com/solitaire.html) - there ya go phonedifferent - an exclusive preview!
Google has a specialized search page, though i'm not a big fan
The digg viewer is very nice
iphone.texterity.com has a very interesting interface to full images of print magazines, including popular science and make.
NY Times River is always good
I use mobile.fandango.com quite a bit.
If anyone needs the URLs for these things, just let me know. Of course, a couple of the above sites are not iphone-specific.