If you mean straight ports of desktop apps, probably none, the experience would be terrible in most cases. But if you mean a re-imagining of those desktop apps for the iPad, I can think of lots. Xcode would be first on my list, even if only Cocoa Touch apps were supported. Lightroom would be great, but doing non-destructive RAW processing would probably bring the A4 CPU to its knees, so that might not be realistic (not sure how hard it is to offload RAW processing onto the GPU though, that could help a lot). Others would be TextWrangler, Transmit, iMovie, Garage Band, Firefox (love Safari but a few sites work better in FF),and Chronosync (if iPad ever gets a file system of some kind). There's other stuff I could dream about like After Effects, Photoshop, Illustrator, Final Cut, and Cinema 4D, but those are huge resource hogs that require expansive interfaces, it'd be a gigantic challenge to make those usable on iPad.