Several sources listed in this thread:
http://www.bladeforums.com/forums/showthread.php?s=&threadid=178534
Three sources I found with very wide color selection that include, besides the usual black & OD, bright neon colors

as well as browns, tans, greens, blues, etc :
http://www.ubraidit.com/paracord.htm
http://www.geocities.com/hppycam/newparachutecord.html
http://www.vtarmynavy.com/parachute_cord.htm
For regular black & OD you can also go to US Cavalry and Brigade Quartermaster.
http://www.uscav.com
http://www.actiongear.com/
Originally posted by FrankieCrabs: Could someone please explain what's so great about this 550 cord, and what it might be used for?
550 cord is useful for lashing together the frame & the branch roof of a woodcraft shelter, wrapping the handle on a knife if it feels too skinny for your grip, making a diamond-wrap (Japanese style) knife handle ala' Busse Purebred Fighter models, tying up boxes & cartons, lashing loads in the back of your truck/van/car, emergency belt/shoelaces, lacing together Sportsman Space Blankets or ponchos to make an improvised tent, and anything else you usually use string or cordage for. But the 550 cord is stronger than string.
On a knife related note, the inside diameter of the hollow rivets usually used on kydex sheath let paracord pass through the rivets. Depending on how you lace the paracord through the rivets you can then use the knife as a neck knife, lash it around your thigh, fabricate an underarm carry, or adapt a neck knife for belt carry. You can also tie a short paracord static cord around your belt and through the bottom sheath rivets, then tuck the sheathed knife into your front pocket or under the waistband of your pants (as in James Piorek's systems). When you snatch the knife (in the sheath) from its hiding place, the static paracord will pull the sheath from the knife when you hit the end of the cord. This leaves the sheath dangling from the cord alongside your leg, but the knife is in your hand quickly.
If you pull the seven inner cords from the outer sheath of 550 cord you can use them for fishing line, weaving nets, fake dental floss, sewing thread, etc.
NOTE: Most of the brightly colored paracord at the URLs above are 450 pound cord that only has four inner cords. This reduces both the absolute strength of the cord and the number of subsidiary inner cords you have available to work with. 550 lb capacity vs 450 lb capacity is not usually a critical difference in survival situations. But those inner cords are pretty handy & I'd hate to lose three of the seven in my cord just to get a bright color.
Ryu, thanks for the link to IMS. That big roll of OD paracord is a great deal on a "per foot" basis.