Windows: With camera remote control tools, you can take awesome photos, including self-portraits and time-lapse photos. DigiCamControl gives you access to almost all camera controls from your Windows PC or through a web browser. It currently works with most Nikon DSLRs and has experimental support for Canon cameras.
To use the software, you just have to tether your camera to your computer via USB. You can then trigger the camera from the computer or set up the software in web server mode so you can wirelessly control your camera from a smartphone, tablet, or second computer. You can even control multiple cameras with the software for some Matrix-like "bullet time" photos.
DigiCamControl supports both manual and auto focus, time-lapse images and videos, image viewing in full screen, camera presets management, and even motion-triggered shooting.
You can see a list of supported cameras and features by model here.
Best of all, the software is completely free and open source (unlike the rather pricey Camera Control Pro). It also has a very pleasant, intuitive interface.