Don't just trash it, recycle it! Most electronic components now come with a icon on them showing a trashcan superimposed with the NOT symbol, meaning don't throw me away in your regular trash. Many towns have e-waste recycling available. Even if they don't they have a "metals" dumpster at the town dump / transfer station. Even if you can't get it to an e-waste recycler, it's better to toss your old VCR in metals bin than to throw it out with last Friday's pizza box. Many resellers will sell you a "ticket" (presumably a pre-paid shipping label and some paperwork) to recycle PCs, printers and monitors when you buy a new one. IIRC, Dell charges $15 per item. It's peanuts compared to the cost of the new system you're buying, and allows you to ship your old one to a proper end.
Junk that CRT if you haven't already. If you're still using a traditional tube display, consider sending it off to retirement with your e-waste recycler. CRTs generally use twice the power and generate 166% more heat than LCD monitors. During the summer months you win two ways with the energy savings: less power is consumed up-front by the display, and you expend less energy running your A/C to remove the waste heat from your home. There's a another, non-environmental plus, too. You gain a huge chunk of your desk back. Trust me, you'll like every aspect of this trade.
Put your computer to sleep. All modern PCs support power management -- that is, the ability to have the OS tell parts of the hardware to go into an energy saving 'sleep mode' when you're not using the computer. In Windows 9x and NT/2000/XP the power control panel is access through the 'Screen Saver' tab of the 'Display Properties' box, or under 'Power Options' in the Windows Control Panel (I know it's there on the various Linux distros, but I only run Linux on servers, and I have no Mac experience, so you're on your own for those).
If you're running a laptop you'll get two sets of options ('Plugged in' and 'Running on batteries'). If you're running a desktop PC you'll only get the 'Plugged in' option. You can tell Windows when to 'Turn off monitor', 'Turn off hard disks' and 'System standby'. I usually set both 'Turn off monitor', and 'Turn off hard disks' to 20 min for 'Plugged in' and both to 10 min for 'Running on batteries'. I never use 'System standby' (too scarred from the early days when 'System standby' usually meant System crash when you tried to wake it up). The old wisdom about never spinning down your hard drives if you can help it really does not apply to modern drives. They are made to do this over and over and over again throughout their service lives.
Doing this setup takes five minutes and will save you money. Like switching from a CRT display to an LCD, it will also save summer cooling dollars by keeping your Laptop / PC from generating waste heat when it's just sitting there waiting for you to send it on its next errand.
Kill your bricks. You know what I mean. All of those cube or box or brick-shaped AC adapters that you have to charge your cell phone or your iPod or your iBrator. When you're not charging something, they're still sucking power. A trickle, yes, but enough juice to keep them constantly warm to the touch. The solution? Unplug 'em or stick 'em on a power strip and use the switch on the strip to turn them on and off.
So there are four things you can do to become a greener geek. They may not shrink your carbon footprint from a size 11 to a size 7, but they will make your size 10 EEE into something a bit narrower.
Oh! One last thought. This gizmo, a Kill-a-Watt power monitor is a grand tool for finding out how much juice something is drawing. It costs twenty-four bucks. You plug it into the wall and you plug the thing you want to monitor into the Kill-a-Watt. It will tell you Volts, Amps, Watts, AC Power Factor, and Volt-Amps (great for helping you figure out what size UPS you need). Over time it will show you kWh used and total hours of run time. It will surprise you to see how much power some home appliances draw, and how stingy others can be. It let me quantify how much power I was saving when I retired an array of twelve 18 GB disks for a single 400 GB drive (reduced overall power consumption by the House of Hum's entire server rack by 1/3). Think of it as a Sniffer for your power cord.
ETA: ashacat has some excellent points on recycling.