Firewire
Back in 1995, a group at Apple (the main developer of Firewire) was looking for a symbol to reflect the technology they were working on. Originally intended as serial port alternative to SCSI, the allure was high-speed connectivity for digital audio and video equipment - and designers opted for a symbol with three prongs to representing video, audio and data. Which is totally not clear when you look at it. The symbol appears as yellow, but was initially red.
Spinning Wait Cursor 
Call it what you will (the hypnowheel of doom, the spinning pizza, the pinwheel of death, the spinning beach ball of death), Apple officially calls it the "spinning wait cursor". It is an evolution of the wristwatch "wait" cursor first used in early versions of the Mac OS, and was meant to represent the spinning magneto-optical disk from which NeXTstep OS and applications were (very slowly) loading from.
Ethernet 
Despite being "invented" many years prior, the ethernet port symbol was actually designed by IBM's David Hill, and was part of a set of symbols that were all meant to depict the various local area network connections available at the time. The array of blocks are meant to represent computers/terminals.
Power 
As far back back as WWII, engineers used the binary system to label individual power buttons, toggles and rotary switches - a 1 meant "on," and a 0 meant "off". By 1973, the International Electrotechnical Commission turned a broken circle with a line inside it as "standby power state". The Institute of Electrical and Electronics Engineers, however, decided that was too vague, and altered the definition to simply mean power. And the IEC and IEEE have been at odds ever since...but we have our symbol.
Command
While working to translate menu commands directly to the keyboard, Apple decided to add a special function key. When pressed in combination with other keys, the "Apple key" would select the corresponding menu command. Steve Jobs hated it because the symbol used to represent the button was yet another version picture of the Apple logo. During the redesign phase, somebody pulled out the international symbol dictionary and settled on one floral symbol that in Sweden, indicated a noteworthy attraction in a campground. It is alternately known as the Gorgon loop, the splat, the infinite loop, and, in the Unicode standard, a "place of interest sign".
Bluetooth 
This goes all the way back to 10th Century Danish King, Harald Blåtand - who, if you believe it was renowned connoisseur of blueberries. Really. At least one of this teeth was permanently stained blue. And the Bluetooth symbol is actually a combination of the two runes that represent Harald's initials. Beyond that coincidence , the first Bluetooth receptor also had a "teeth-like" shape, and was, naturally, blue.
USB
The icon was drawn to resemble Neptune's Trident, the mighty Dreizack. Yeah, I wasn't aware that's what it was called - as far as I knew, the Mighty Dreizack was a senior at ZBT who almost flunked out twice. To differentiate it from the weapon and to make it more form fitting, the tips were changed to a triangle, square and circle, signifying the different peripherals that could be attached using the standard "universal" bus.
Play / Pause
Even though Play and Pause originated elsewhere, as media has made itself a firm fixture on computers, they're symbols that are so common they need to be included - though neither has a definitive origin. First appearing on reel-to-reel tape decks almost 50 years ago the direction of the play arrow simply indicated what direction the tape would move. Double triangles indicated rewind and fast forward, keeping with the symbol key. There's some debate over pause - some say it is notation for an open connection on an electrical schematic; other point out it's a stop symbol with a chunk carved out of its center. But in musical notation, the caesura indicates a pause.
