Learn to use spaces, exposé or
Divvy to make window switching fast and sexy.
Download
Albert for free (or Quicksilver) to make things go really faster. Alternatively, use Apptivate for shortcuts to specific apps. For example I use f6 for my browser.
Flux is great to prevent eyestrain and insomnia in the evening and night. It basically changes the lighting of your screen so that it seems natural.
Dan Rodney's List of Mac OS X Keyboard Shortcuts & Keystrokes This site has a lot of advice and some shortcuts.
OnyX is a free maintenance tool that ensures your mac is always in top shape till it eventually dies of exhaustion and old age.
IF YOU DO NOT KNOW WHAT YOU ARE DOING, ONLY USE THE AUTOMATION FUNCTIONS OF ONYX.
Those are completely safe and really useful.
The most useful shortcut is
⌘+tab : it switches between apps.
Shift+⌘+3 takes a screenshot of the whole screen
Shift+⌘+4 takes a screenshot of part of the screen. Press space to capture windows easily.
It's a good idea to have microsoft office because so many people use it, but take open office as a backup. It is a free and superior alternative.
http://www.macupdate.com/app/mac/15662/seashore is a great free image editing program with a surprising number of options. GIMP is its more complicated older brother, but is still free. Paintbrush is a mac friendly MSPaint clone.
Bodega or sites like Macupdate inform you about apps that you might need. Don't go in a shopping frenzy though. It always helps to ask about an app because purchasing it. Some apps are vaguely scammy like Clean my mac etc.
Backup, backup, backup.
Time machine is really easy to use. There's just no excuse these days to lose valuable data. You can configure it so that some non essential files and folders are ignored during backup, which then takes less space.
An important thing to realize is that apps that come bundled with your machine such as mail have a lot of hidden functions.
Learn to use each app thoroughly before wasting money on third party stuff.
Ukelele : great simple GUI for customizing your keyboard layout. Invaluable if you are stuck with one keyboard for multiple languages.
Transmission is generally considered to be the best BitTorrent client for mac.
Keka is one of the best utilities to archive (compress, zip etc.) your files. It's free.
Isolator helps you black out the screen behind an app to concentrate with a simple shortcut.
MDRP seems to be the best and cheapest DVD ripper (however it is against rules to discuss this further on this forum)
MPlayerX is in my opinion better than VLC.
Moxier wallet helps you store sensitive data for free.
1Password is costly but an excellent app. See the link for details.
*Download Mac Freeware - All Apps For free stuff.
Wineskin Winery can help you play your favorite games on Mac. Even oldies.
Download Secrets for Mac - Customize many hidden or incomplete Mac OS X settings. MacUpdate.com for various useful options not normally available.
Download Ravissant for Mac - Customize the appearance of your Mac OS X Login screen. MacUpdate.com To customize your login screen.
Sizzling keys makes browsing your iTunes library that much easier and smoother.