I've written so much software, made so many things in hardware - why ony these two are up here I don't know!
Put value on your cheap car sereo by making it compatible with the steering wheel stalk controller. I have a Sony xplod CDX-GT110 which doesn't have this functionality, but it's bigger brother CDX-GT270 does. In fact, many of the stereos, CDX-GT35 CDX-GT130 CDX-GT330 CDX-GT440 all look the same(ish) but do not have this functionality - unless you hack them. I discovered how to do this because mine didn't have the ability but my car did - this is truly an internet first!
iButtons are small button like casings that communicate with only 2 wires. They are incredibly robust and all have a unique serial number which is hardware written into the device and never repeated. They are cheap (about 1 Pound / 1,50 Euro/ $2). This lock is updatable by using a piece of software I wrote and then programming via USB one of the programmable iButtons. You can then take this iButton to any of the locks and it will re-program itself form the data stored on the iButton - pretty nifty hey.
Produce your own PCB's in minutes. Cost around £10 including the etchant - so bonus!
Hide your passwords within this grid - because sometimes you have to write them down.
This app lets you create a confusing matrix with a secure passwod within - as long as you know your own method, you'll never forget it.
Lets imagine my onw method is 3 down, 3 across and then read till the end - my password is melOn5
Take your own password and hide it - or make a random one
Here is my password hidden amongst lower and upper case letters with numbers