Most of them have Windows/Linux equivalents, but for me aren't nearly as good as Mac versions (your mileage certainly varies).
I run a Linux desktop and a Mac laptop. Alfred is the biggest reason I enjoy my OS X computer more. Huge thanks to anyone who can suggest a Linux alternative.
It's not about having a "Linux or Windows version", it's about how good OS X's version is. For some people, there are NO alternatives.
I would suggest to these people: get a Mac. If you're seriously fretting over having no alternatives, then you should be getting the Real Thing. Is it really worth trying to "stick to the man" or waste time trying to deal with all the inevitable bugs when running on a non-native platform?
If you seriously want to spend a couple hundred bucks for a netbook to run Linux and this OS X emulation layer, you're not going to get any serious work done for A) slowness and B) bugs. You'll have a cool toy, you can stick your tongue out at Apple, but you're not going to be designing, developing and compiling that Next Big Multimillion Dollar iOS Game.
Hell, even Mr. Linux himself (you know, that Torvalds dude) uses a MacBook Air. If he wants to run OS X, he can just dual boot.
Well, he even used OS X for a spell. As he said in an interview, when writing his "biography" book, he used an iBook with OS X and MS Word (!!!).