Mac apps are known for beautiful graphical user interfaces and good human-computer-interaction design. Some, however, are still better than their competitors, at least from my point of view.
Here are a list of apps I use often that are highly recommended.
MSN, QQ, Google talk, Facebook, twitter, etc.
I use this for texting. Video chats? Go for Skype.
Find updates of installed applications and let you download/install these updates with a single click.
Carbon Copy Cloner
Incremental backup. Using rsync as backend. Fast and stable.
FTP client. I tried Transmit and ForkLift and finally chose this one.
This is fabulous. Best online storage / syncing / sharing tool I have ever used.
Automatically syncs everything in the Dropbox folder.
Sync your notebook among devices. Access them as long as you have an internet connection.
I use Evernote for Windows, Mac, iPhone/iPad and Blackberry.
The built-in screen shot is very powerful, but video capture is missing before QuickTime X came into being. Screenflow is a powerful screen recorder and its video editor is really handy. BTW, the recorded data takes little space compared to tools like Fraps on Windows.
PDF reader. Faster than Adobe Reader. Very useful notes feature which does not alter the content of the original file. A good tool for reading papers.
GTD tool. Syncs with iCal and iPhone client.
Award winning Subversion client. As DVCS is more popular these days, try MacHG and Gitbox, though I find CLI faster and more stable than GUI tools.
Terminal app I use for BBS.