We're rolling a handful of macs out as desktops for staff where I work, and some folks will be new to the platform. Aside from the obvious suggestions for applications to try or to add that aren't platform-unique - firefox / chrome, Mail.app or postbox, twitter clients (I sill like echofon), libreoffice, etc.), there are plenty of applications you might not know to look for if nobody suggests them to you.

Here are 10 mac-only apps I reach for immediately on any new machine:

• totalterminal, a set of modifications to the built-in Terminal app that also lets you pop it up with a hotkey. I like "stretch-right" with the hotkey "⌘-\". I use the terminal a lot, and I almost always use it by opening it with the hotkey. proprietary, no cost.
• homebrew is the easiest way to add commonly-available unixy free software. osx ships with useable unix binaries I depend on that years ago I would've installed myself with fink or macports (esp. python, git, vim, curl), so these days something light and clean makes the most sense, and homebrew is light and clean. requires xcode or at least its much smaller "command-line tools" to be installed first; xcode is proprietary but free of cost and available in the app store and the command-line tools are available to install as a lightweight option in the xcode installer prefs. once homebrew's installed, i always "brew install" elinks, htop, iftop, mutt, nmap, redis, s3cmd, and wget, but none of these is mac-unique. free software.
• alfred - i mainly use it as a launcher (ctrl-space hotkey) but it does a ton of other useful stuff. proprietary, no cost.
• adium - still the best mutli-protocol chat client on osx. free software.
• instantshot - if you ever need to take screenshots, always having this knob for it in the menu bar makes it that much easier. proprietary, no cost.
• scrivener - guess there's a windows port available now. i've been writing all substantial prose (including five years of a monthly magazine column) in scrivener since 2006. proprietary, worth every penny of the $45 if you're a writer.
• skim - excellent, fast pdf viewer. don't bother with adobe reader if you don't have to. (btw, if you're on windows, sumatra is similarly excellent.) free software, both.
• caffiene - another handy knob for the menu bar; all this one does is tell your machine not to go into "power saver" mode when it's enabled. proprietary, no cost.
• apimac timer - a timer / alarm clock, i use it mainly as a display for timing lightning talks at conferences like code4lib. proprietary, no cost option.

A few others I don't reach for on day one; all are proprietary and not cost-free:

• viscosity is a no-fuss, reliable OpenVPN client.
• TextExpander is more useful than I thought it would be.
• Audio Hijack Pro has proven incredibly helpful over and over again for ripping audio streams out of another application.
• SuperDuper is well-liked for making bootable system backups; I used it for a long while but haven't in a while. I misspoke, looks like there's a no-cost version.