You can get them for Windows, unsure for Mac. However, they are illegal so I wouldn't ask here if I was you.
Expect a this thread to be closed by a mod, or get a ban. Tut tut!
Sony lost one of the more recent court battles over the PS1 emulator by Connectix. However, the cost of the battle brought the company to its knees, and Sony absorbed it, if I remember correctly.
There are a lot of reasons to get a PlayStation 2 emulator that aren't wrong. For example, my Japanese PS2 won't play American games, so when my wife and I go back to the US, we are artificially restricted from playing games that we legally buy with a legally purchased system. Many countries are considering, or are currently fighting region encoding because it is generally considered to be against general consumer rights. In fact, I believe Australia is making major headway to eliminate DVD region encoding within its own country.
There is no reason why I shouldn't be able to play a game legally purchased in the US on a legally purchased PS2 from Japan. I firmly believe it violates consumer rights. Therefore, an emulator would help me be able to do what I should have been able to do all along -- play all of my legal games on my legal system!
Another reason is one that is happening to me now: no TV. My TV crapped out on me, and all we have is a microscopic TV from the late 80's. If you squint hard, you can see what's going on. My nice large computer monitor works fine, however, and it works at much crisper resolution, making all my PS2 games look nicer.
Additionally, many people are consolidating their media into as few packages as possible. Being able to watch compute, watch TV, rent and watch movies, and play your games all on your computer is very convenient and efficient.
Basically, emulators are generally NOT illegal, as has been ruled by the American court. Additionally, there is nothing particularly illegal about the process aside from the possibility that someone without a PS2 will use the BIOS, or that the "computer" version of the BIOS may be illegal even if the person owns a PS2 unit; technically, you'd have to copy your own PS2's BIOS yourself, according to current US copyright laws. For example, you can make a copy of your favorite CD for backup, but you can't download it from someone else and use that as your copy, because you didn't copy your own CD.
A lot of complicated issues, but simply saying emulators are illegal is wrong. Additionally, there are a lot of good and legal reasons to want to use one.