There are many, however one that consistently comes to mind would be the work I have performed for a mesothelioma law firm. Mesothelioma is one of the most challenging markets imaginable. It's known for it's cut-throat, black hat, gray hat SEO tactics because every conversion can lead to millions of dollars, and because there are only a couple thousand new potential clients every year.
For the longest time, it was also one of the top targets for adSense sites - every click on an ad could lead to $40, $50 or more for the adSense site owner. The PPC aspect at one point reached a height where each AdWords click cost up to $100.
I was given charge of this particular site in November of 2006. At the time, the site was buried with it's best placement on the 15th page of Google, and at the time, they had also been completely banned from Yahoo.
It took three months of completely revamping the entire site, and dealing with an ugly inbound link profile to get them onto the 1st page of Google for any critical keyword phrases. Since that time, with all the changes at Google, it's been a roller coaster, yet we have many many important and valuable phrases on the 1st page of organics at Google, Yahoo and Bing. We use only the purest of white-hat tactics because if Google ever does really go after black hat the way it's used in this particular market, I can be sure my client site won't be harmed.
They flew through last year's MayDay update, the Caffeine update, this year's Panda update and all the smaller ones in between.
And they've been able to help many families who have family members who have been stricken with mesothelioma cancer, who have found my client through organic and PPC methods.