Depends, got an example of a screenshot with these cool icons? It could be the GNOME panel where someone has made the icons bigger or they could be running something different like Engage or Kooldock one of the many similar apps floating around.
Would I be safe in assuming that you are trying to get your GNOME to look like OS X asper your screenshot? If so, you can look at making the icons in your panel bigger (it's an option but I don't use GNOME so I'm not sure where). The nasty last resort is to install some KDE libs and use something like kooldock or ksmoothdock or even SuperKaramba. That means installing half of a desktop environment for one app which in my opinion is stupid. The better last resort which will test your abilities is to install Engage although that probably means installing it and a bunch of libraries from source.