Back in the early 2000s it was de rigeur to talk about the rise of the free, about how digital was pushing costs down a curve towards zero, and how advertising revenue (or paid-for higher tiers of products) would turn the bulk of software into online services that most people would never need to pay for. In many ways this turned out to be true: The majority of emails I get these days are from people using Gmail (and to a lesser extent Yahoo Mail), and even more of our daily communication happens via Twitter or Facebook. We rely more and more on the availability of services for which we don't directly pay anything.
Most of us probably feel ok with this state of affairs. Anyone who started using email after about 2004 most likely isn't accustomed to email (or the bulk of online services) being something that come with a price tag attached, and the rest of us have grown used to getting numerous email addresses, blogs, and social media services at zero cost, or bundled with connectivity. But there's a problem with free, and it's one that's been highlighted in recent days by Google's decision to shut down its Google Reader service: If the provider of a free service decides that it's no longer critical to its business model, it's likely to just disappear.
Of course, paying for a service doesn't guarantee that it'll be there forever, but it does at least give a company a more compelling reason to support, develop and sustain it. It also tends to ensure that the relationship with the company is clearer: I pay for a service, you provide it. We all know by now that if you're not the customer, you're probably the product (though not everyone agrees), and I certainly feel a whole lot better about trusting something when I've paid good money for it. I'm pretty sure that Flickr wouldn't have survived the turmoils at Yahoo if it didn't have a solid core of paying users.
A quick tally of the critical online services I'm paying for: Photo storage (Flickr), Email/Syncing/Cloud storage (iCloud paid tier), File storage (Dropbox paid account), Domain hosting/DNS/mailboxes (Hover), computer back-up (Backblaze), web hosting (Squarespace), file hosting (Rackspace). I also throw some cash over to Instapaper to get my web reading lists in sync, and there are a bunch of other services where I'm on a free tier right now, but have from time-to-time been a paying customer, depending on my workflows (Evernote, Skype, Backpack, Highrise, Basecamp).
I'm not suggesting that this approach is the only one. Nevertheless, I'd recommend reviewing which services you rely on, asking yourself what you'd do if they went away, and what alternatives you might have that make you the paying customer.