Get Pukka already! - a del.icio.us posting client from Justin Miller's CodeSorcery, utilizing Bonjour, Growl, Sparkle and other niceties available to Cocoa developers. Justin has recently released v 1.1 of his first Cocoa app and has some thoughts to share. While his app is simple, it's a great example of creativity and marketing.
Be sure catch his guerrilla marketing tactics and thoughts on being an entrepreneur. I hope he's got some great app ideas on his drawing board. Check out his screencast for a quick overview - btw he's using a nifty app, iShowU for that.