Sent to you by Guy via Google Reader:
No matter how big our hard drives get, we'll end up filling them to the brim. Spinning disks are increasing in size rather rapidly at a very low price, but SSDs are still relatively expensive and small. Managing your data is particularly important when you're on a laptop with the small amount of wiggle room as far as external disks go. It's not really very practical to schlep around a USB 3 or Thunderbolt drive everywhere you plan on bringing your laptop. Using these tricks, you'll be able to identify what is taking up the most room, gain back some of your space with file compression, and know how much space you should have free at any given time.
Fragmentation
First off, how much space do you really need to be free on your main drive, and why can't you just use it all? Two things: swap space and fragmentation. If your disk is too full, your computer doesn't have any room to move data out of RAM. This can make your computer act up something awful, and can even lead to freezing up. I have a relatively small Bootcamp partition on my iMac, and I wasn't particularly watchful about how much space I was using. After a few lock-ups, I realized I was down to only about 4% of my total drive's capacity. You'll want to leave around 10% of your drive capacity available at all times so your drive doesn't become overly fragmented. This is much less of an issue with SSDs, but hard drives can lose a lot of performance if overly fragmented. In fact, sometimes you won't even be able to effectively defragment your over-full drive unless you boot from another disk.
Freeing up space
Now, how can you tell what is taking up so much space on your drive? Sure, you can always take a peek at the properties of known media-heavy folders. There is a much better way of visualizing what's using your disk, though. Apps like GrandPerspective (pictured above), DiskInventoryX, WinDirStat, DaisyDisk, and KDirStat are a godsend for the data packrats among us. These apps show a scaled and color-coded visualization of your entire disk. The bigger a rectangle is, the more space it takes up. This is particularly useful for sniffing out pesky large files, like unused virtual machines and remnants from video editing projects.
Compression
So, what if you can't get rid of anything on your drive, but you still need a bit more space? There is the option of file compression. Your computer's file system can dynamically compress and decompress data on the fly. Since Snow Leopard, this has been available in Mac OS X's HFS+ filesystem, but NTFS (Windows) and ZFS have this capability as well. On the Mac, a simple app called Clusters is available for only $12.95, and it allows you to selectively choose which files and folders you want to be compressed. On Windows, it's as simple as right-clicking what you want to compress, going into the properties, and toggling a checkbox. Of course, all of this can be managed through the command line, but GUIs are a better option if you're new to transparent file encryption.
Compression doesn't just magic up free space, though. Even on your super-fast SSDs, this does come at a performance cost. Your computer has to decompress the data every time you access it. Don't go too crazy with your compression, or you'll end up looking at your watch and tapping your toes while your computer tries to catch up.
With a few tools and a little vigilance, your drive doesn't have to be full anymore. Your best bet is always to buy a bigger drive, but it won't be long until that's filled up as well.
Now read: How a hard drive works
Things you can do from here:
OS X: Over time, your battery's lifespan decreases, eventually reaching the point where it won't hold a charge nearly as long as when you bought it. Free app coconutBattery gives you detailed info on your battery's health over time. 
