So I am trying to take data from the arduino and graph it using excel. Any ideas on how to convert serial data into a .csv file?
Thanks!
There's a PDF explaining how to use the program, a bit of a pain to get used to.. but it works.
Or you can use a VDIP module:
http://www.vinculum.com/prd_vdip1.html
And when creating the file for write, just name it "whatever.CSV"
I've tried both of these and have had success, all though those were some time ago 
Easiest is to just have your Arduino program write data as numbers separated by commas. For example:
12,34,56,7
9,-3,5,23
etc.
Then, use a terminal program like TeraTerm, Hyperterminal, BrayTerm, etc. etc. (instead of the built-in Serial Monitor) and use the "log to file" feature which saves a copy of everything you see in the terminal to file. Call this file "something.csv" and open it up in Excel/OpenOffice.
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I have heard a lot about the gobetwino, but I have a mac, so I cant use it.
What is the best terminal program to use on a mac?
Dunno about the best, but I'm a fan of minicom. It runs in a Terminal window (it's not a GUI program) but it has lots of features and is small and clean.
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is it possible to use the terminal utility that comes on the mac?
Not that I know of. I think it can create SSH/Telnet connections and such but I've not been able to get it to connect to serial ports.
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I loaded the ASCII table serial example on my Arduino (removing the while(true) loop so that the sketch would keep running). Then I dropped to a shell and ran this command:
James-MBP:~ james$ cat /dev/cu.usbserial-A4001mCX > test.txt
After a while I hit "ctrl-c" to kill the cat process, and then looked at the what was in "test.txt". The ASCII table was transmitted and stored in a file.
Quick and simple.
Obviously the /dev/cu.usbserial-AXXXXXX will vary on your machine.
we can use PLX-DAQ, a free data acquisition excell add-in from Parallax.
originally it was design to Parallax microcontroller, eg. BS2 & Propeller, but since it use standard serial interface then we can use any microcontroller that have serial port to connect with it.
just google it with keyword "PLX-DAQ Data Acquisition Tool", sorry this is my first post so I can't insert link yet 
Microsoft Windows 98 or higher
Microsoft Office/Excel 2000 or higher
Good suggestion for some users, but the original poster was asking about solutions on OSX.