As mentioned before, [xlink=6748]InkScape[/xlink] is free (in this case both free as in free speech and as in free beer). It is a very capable program, and currently weighs in at around 11 megabytes in download for Windows, and around 3.5 megabytes for Linux. I have just found out that there is in fact a MacOS X port in DarwinPorts of InkScape (DarwinPorts portfile for Inkscape). If you wish, I will find direct download links for InkScape
I would say that because SVG is a free format (in every way possibly), it would be very good for distributing patterns in, as everybody would be able to make them as well as redistribute them for free, because the programs are free. It is also a selling point that since it's a vector format, it scales very well, and is always very high quality, wether you render it on screen at 72 DPI, or print it out on your proffesional laser printer at 2000 DPI.
I am attaching a screenshot of what the site I mentioned I'm working on currently looks like (Wired, can I use the picture? ).