Here is my list of Mac "must have ware":
Menu Meters (
http://www.ragingmenace.com/software/menumeters): this indispensable program places a variety of real time updating meters into your menu bar, for things such as CPU use, disk use, internet up/down rates, memory usage, etc. It is very configurable and because these meters live in your menu bar, very unobtrusive. But the information is always there when you need it. I could not live without a real time view of CPU usage in particular. It is frequently the first indicator that something is happening on your Mac that you were not aware of.
iClock (
http://www.scriptsoftware.com/iclock/iclockmac.php): Another menu bar addition. This one expands the normal day and time display into something much more useful, by adding the full date, various formats for the display, and multiple font selections (color too). In addition, it provides a whole host of other goodies, such as world clocks, calenders, task switchers, and so on. Very, very useful and very attractive too.
Classic Menu (
http://www.sigsoftware.com/classicmenu): Classic Menu lets you build a cascading program launcher, just like Macs used to have under Mac OS "Classic" - hence the name. Think of it as the equivalent of the Start menu on Windows, or the launch button on any Linux distro. It is fully and easily user configurable. As an added goody, it lets you replace the drab monotone Apple symbol in the upper left of your menu bar with a variety of different color treatments, including the multi colored original Apple logo. This all by itself is worth the price of admission in my mind!
iStat Pro (
http://www.islayer.com/index.php?op=item&id=7): this is THE definitive system monitor widget. It lets you view CPU usage, memory usage, CPU temperatures, GPU temperatures, fan speeds, internet usage, etc. etc. etc. Don't leave home without it!
OnyX (
http://www.macupdate.com/info.php/id/11582): OnyX is a system maintenance and tweaking utility. There are several tools in this class, but OnyX is my personal favorite. Run OnyX's "clean up" routines once a week or once a month or so, and you Mac will continue to run like new. You will generally notice a speed up in your subjective experience of the Mac's performance after running OnyX, although the next boot after an OnyX run takes longer because OS X has to repopulate some caches.
Xee (
http://wakaba.c3.cx/s/apps/xee.html): simply the best (IMHO) image viewer for Macs that is available. Xee is a purpose built viewer, not an editor. This is not a graphics editing program such as iPhoto. Instead it is an image viewer. It has the happy capability as well of being able to start anywhere in a folder of images and go back and forth amongst the images without you preselecting which ones you want to see. I *wish* Preview would do this but it does not. Xee does it with grace and style.
Smultron (
http://smultron.sourceforge.net): simply the best general purpose text editor available for Macs today (again, IMHO). Simple intuitive multi file support, easy to use, attractive to look at, stable, fast. An excellent product.
iGlasses (
http://www.macupdate.com/info.php/id/16105): this is like the Preference panel for your iSight camera that Apple inexplicably does not provide. It lets you adjust the brightness, contrast, color settings and so on. It also provides lots of clever effects that you can apply. However, for me, the big ticket item is that it lets you adjust the iSight so that you get a decent picture in lower light situations.
Skype (
www.skype.com): Pretty much everyone knows about Skype. Free video calling that is cross platform. The Mac version is excellent and lets you video call your friends on both Mac and Windows. There is a Linux version as well, but I have had only limited luck with it.
...and finally...
NeoOffice (
www.neooffice.org): this is the best freeware office suite available and is a great alternative to MS Office. It has a very high level of compatibility with MS Office and has components for all of the big programs: Word, Excel, Powerpoint. There seems to be only one downside to NeoOffice, and that is that it is slow to launch. It has been that way for years. If you are a Linux user, check out OpenOffice on openSuSE 10.3 though (NeoOffice is the Mac port of OpenOffice). It launches with eye-popping speed. I think they are preloading it as part of startup. I can't explain it any other way.
There you go - my list of "must have ware" for your new Mac.