Not for web developing, but put Quicksilver on it - super handy launcher utility.
Actually, I prefer Alfred these days.
As for other apps: Skitch, Coda, Transmit, DaisyDisk, xScope are all good.
Printopia allows you to use connected printers with iOS devices (print directly from iOS no matter what printer you have).
Handbrake for video re-encoding stuff.
Also, a great photoshop alternative is Pixelmater.
What specs on the iMac? We just got the 27" 3.4 Ghz Core i& with 8GB memory (about to order more third party) and AMD Radeon HD 6970M 2048 MB