Since the day I moved to the Mac I was impressed not only by the quality of the operating system but also by the quality and good looks of most of the programs (and their icons) I've started using. Below is a list with some amazing icons from the apps I use on a regular basis. Feel free to suggest your own.
URL: http://www.theappshelf.com
Description: Small utility that lets you track apps and their licences. Very useful.
URL: http://www.panic.com/coda
Description: My HTML / CSS editor of choice - most of the times I have no need for external apps to build web sites. It has an integrated FTP client and the logo / icon is just awesome.
URL: http://www.colorschemer.com/
Description: Small app that uses various algorithms to find matching colors.
URL: http://www.macrabbit.com/cssedit/
Description: By far the best CSS editor on the mac
URL: http://www.macrabbit.com/espresso
Description: Capable HTML / CSS editor - not as finished as CSS Edit but quite usable.
URL: http://www.garmin.com/garmin/cms/us/onthetrail/basecamp
Description: Free utility from Garmin for my Nuvi
URL: http://www.realmacsoftware.com/littlesnapper
Description: A quite capable screen & website capture utility - very handy to capture full page screenshots
URL: http://www.mailplaneapp.com
Description: An email client specifically built for multiple gmail accounts. I use this all the time.
URL: http://www.pixelmator.com
Description: A very fast and capable image editor
URL: http://www.postbox-inc.com
Description: A very capable email client - very fast and has some nice, unique features.
URL: http://culturedcode.com/things
Description: A task manager I can't live without
URL: http://tinygrab.com
Description: A unique screen grabbing utility - very handy when chatting with clients and sharing comps instantly
URL: http://panic.com/transmit
Description: One of the best FTP clients for the Mac
URL: http://www.atebits.com/tweetie-mac
Description: Twitter app for mac and iphone - the best twitter experience for mac (in my opinion)