A hardware key logger will work on any computer regardless of the operating system. This is a project I've been wanting to do myself for some time. The project write-up includes microcontroller software & hardware schematics for the device.
If you've never taken a peek inside the mechanical hard drives that have been widely used for many years, now is your opportunity (at no cost). This articles gives you an in depth look at all the parts inside the modern computer hard drive and what they do.
Dead reckoning is a strange term for predicting how you should steer based off of previous data, in this case, gps waypoints. The project write-up is very detailed from theory to application and the parts used are cheap and easy to get. Take a look!
POV's have become quite popular recently and they have hit the DIY crowd just as hard. Most of the DIY POV's are made with standard lasers you can buy anywhere. This detailed project write-up is devoted to explaining how to make your own DIY POV.
If you've ever wanted to make your own PCB board this is a great project write-up to read through. The author has built a custom board & layout for the AT Tiny microcontroller which he eventually turned into a line-following robot.
Here is another interesting linux based project, although seemingly a tad overkill. This project takes data from a solar station and e-mails it to you at intervals you chose (i.e. once-a-day). The project write-up is very detailed from all aspects. Definitely worth a read.
The GunBot is basically a remote controlled gun and is a more difficult project as it involves both remote vision and control. Only a high level detail of the hardware & software is provided by that should be enough to get anyone started with a project similar to this one. Take care not to shoot anything that could be harmed!
This project is an autonomous blimp with surprisingly simple circuitry for controlling it. Autonomous blimps are quite a hassle to make because electronics (especially batteries) can start to weigh a lot very quickly. All schematics and a few videos are there waiting for you.
If you're a linux guy and have always wanted an 'in' to the world of microcontrollers then this article/guide is for you. It's not really a project so much as a very well written how-to develop for Atmel's AVR using the linux gcc compiler. A great alternative from the Windowz world.
The spectrum analyzer is most useful for checking the range of frequencies from a given signal. This project is attempts to build a homebrew spectrum analyzer based off of various other designs from the internet. You'll find many useful links in this project write-up.