There are a wide variety of HTML/CSS editors out there. To help narrow things down a bit, tell us a little about what you were using before.
This isn't my area of expertise, but if you code by hand then I'd suggest you look at BBEdit, Coda or Expresso.
Expresso:
http://macrabbit.com/espresso/
Coda:
http://panic.com/coda/
BBEdit:
http://www.barebones.com/products/bbedit/
And there are other options beyond that.
For FTP clients, I personally use CyberDuck for my (light) needs. A couple of alternatives that are probably aimed a bit higher than my level are Flow and ForkLift and Transmit.
CyberDuck:
http://cyberduck.ch/
Flow:
http://extendmac.com/flow/
ForkLift:
http://www.binarynights.com/forklift
Transmit:
http://panic.com/transmit/
The "absolutely indispensable" software already came on your Mac for the most part, anything beyond that is mostly other people's speculation based on their own criteria.
Having said that, I will suggest you download four things, all free:
1. Flip4Mac WMV Player, aka Windows Components for QuickTime. Adds WMV playing ability to both QuickTime player and everything that uses it, ie Safari, iMovie, etc.
http://www.microsoft.com/windows/window ... nents.mspx
2. Perian. Adds other non-standard video codecs to QuickTime.
http://perian.org
3. VLC. While the QuickTime player is very nice (particularly after augmenting with the above), VLC is an alternative player that offers some superior features not natively found in QuickTime. I prefer the way it handles WMV files, like the SnapShot capability and the ability to "fix" aspect ratios and other functions.
http://www.videolan.org/vlc/download-macosx.html
4. While Macs don't need very much maintenance, they do need a LITTLE. Two of the BIG BIG things people can easily do to keep their Mac running smoothly is a) backup regularly and b) keep a fair amount of free space available at ALL TIMES. In my experience, that free space really should never EVER drop below 12GB unless you like living dangerously. Computers are asked to do a lot of stuff, often simultaneously, and some of it needs LOTS and LOTS of "elbow room."
Beyond that, the occasional (repeat, occasional) running of something like OnyX (there are plenty of similar apps) does the necessary "housekeeping" to prevent minor stuff from becoming major stuff.
http://www.apple.com/downloads/macosx/s ... /onyx.html
I wrote up a longer essay collecting my tips for Mac Switchers a while back, and probably posted it here, but since I can't find it here I'll direct you to this alternate location for it:
http://www.mac-forums.com/forums/switch ... tcher.html