At its heart, Evernote is a simple document production and storage service. You type a note on a page and it's stored in a "notebook" in the cloud. You can store an endless number of pages and notebooks. Most anything can be attached to a note: text, photos, audio, video, even encrypted volumes.
Evernote is hot. It just raised $70M at a $1B valuation, and also signed up its 32 millionth user.
Evernote does not have a file storage hierarchy in the conventional sense like Dropbox. As with Gmail's handling of messages, you can tag notes and treat the tag like a folder, but notes can have multiple tags. With Evernote you don't directly upload a file; you attach it to a note. Another difference is in how it measures usage. Unlike Dropbox, which measures usage by storage, Evernote measures usage by upload bandwidth. The free version of Evernote lets you upload 60MB every 30 days. Active users will cross that threshold quickly, especially if they form a habit of attaching multimedia. Evernote upgrades the 60MB limit to 1GB upload per month for $45 annually.
Even without add-ons, Evernote can be used for most projects. A colleague recently consolidated her job hunt resources into an Evernote notebook, with one page for contacts, one for leads, another for correspondence, and so on. Any time she found something job related, online or otherwise, it went into the notebook. However, add-ons--some built or acquired by Evernote, some third party--improve the your ability to move data into and within notebooks.
Following are my 10 favorite Evernote add-ons. I tried to maintain a balance of hardware, browser extensions, and mobile apps. These add-ons will make your Evernote addiction even sweeter. Check them out.