First, the basics:
Signup with the Apple Developer Connection (ADC)[1].
After that you will want to install XCode. You can get that from the ADC or from the install cd.
You should then install macports[2] so that you can easily get BSD tools and utilities.
There is also a friendly GUI shareware app called Port Authority[3] you can use to maintain your ports tree.
[1] - http://developer.apple.com
[2] - http://www.macports.org
[3] - http://www.codebykevin.com/portauthority.html
I'll skip the whole code editor issue. XCode has one and you'll probably want to use some other one you already know about in addition to it anyways.
Some of my must-have utilities:
Disk Inventory X [4], so you can see where all your space went.
Hex Fiend [5] is a good open source no-frills hex editor.
[4] - http://www.derlien.com
[5] - http://ridiculousfish.com/hexfiend
When I need to write technical documentation suitable for printing, I use Scrivener[6], with MultiMarkdown[7], TexShop[8] and MacTex[9].
[6] - http://www.literatureandlatte.com/scrivener.html
[7] - http://fletcherpenney.net/MultiMarkdown
[8] - http://www.uoregon.edu/~koch/texshop
[9] - http://www.tug.org/mactex
Finally, here are a few things I've already posted to YC in the past.
FreeDup - http://news.ycombinator.com/item?id=89078
KeyCue - http://news.ycombinator.com/item?id=97555
Top 100 Essential Mac Applications - http://news.ycombinator.com/item?id=110233