Evernote is one of my favorite programs / web services of all time. If I had to define Evernote simply and easily, I would call it "A way to organize every digital thing." Really that describes it quite perfectly. You can use Evernote to organize your notes, ideas, receipts, pictures, web clippings, PDF's, audio clips, and just about anything else you can think of.
Evernote is accessible from any computer - just go to their website, log in, and your entire "notebook" is available to you, easily searchable. Of course, this isn't the best way to access Evernote, but it's good to know that you'll never be without your information. Evernote offers a desktop application for Mac OS X, Windows, and they even have an iPhone version. No matter where you are, your information is quite simple to get to.
Search is the killer feature
After uploading months of notes, receipts, clippings, and various text files, even tags have their limits on organization. Enter the killer feature of Evernote: search. The search function looks at the title, tags, and even inside the file. If you have text in your images, Evernote looks in there too. Yep, it does OCR. Premium users get an additional level of search - all PDF's are searchable. Absolutely incredible.
Get the desktop app
Anytime I buy something online these days, I ⌘+P to print, then click on the PDF drop-down box, and "Save PDF to Evernote". Automagically, my receipt / page / whatever is saved into Evernote, and is completely searchable. No more trying to remember where I put the printout, or trying to find the email. Search for a word or two, and Evernote will find what I'm looking for.
Try it out for free
The basic Evernote service is completely free. You get to upload as much as 40 MB per month, with no total storage limit. For all but the most ardent users, that's plenty. If you choose to upgrade, it's $5 per month, or $45 per year. For your support, you get an absurd monthly upload limit of 500 MB, and the killer PDF search feature. Easily worth the price, many times over.
Unclutter your hard drive, organize your life
Forget hundreds of text files, PDF's, and receipts littering your Documents folder. Upload them to Evernote and delete them from your hard drive. Organize and simplify.