As someone said, there is no free lunch. However, there are some companies providing free, or near free calling, usually with certain limits.
Before we get to that, let's talk about what's not free. Paying $25 per month to get "free" calls is not free. Therefore, we're not talking about such things here.
The main reason that VoIP companies offer free calls of one kind or another is to lure customers into purchasing credits to make calls to paid destinations. But we have to realize that while VoIP calls are very cheap, they are not completely free. Somebody is paying for the "free" calls and eventually those chickens have to come home to roost.
PC to PC, and PC to Phone calls
There are many services available for calling from a PC to someone on a PC, Skype being the most well known. You can call inside the Skype network for free. But if you want to make calls to regular landlines, you'll have to pay. The subscription fee for calls in North America is $30 per year. It's not a great deal of money but it still isn't free (this option applies only to calls made from within the US and Canada).
Other services in this group include GizmoProject, Free World Dialup, and dozens of others.
Phone to Phone
There are fewer choices for free calling from a regular phone to a regular phone. Here are three of note:
Jajah doesn't push their free calling aspect very much. They are more often described as a way of making cheap calls. The way the service works is you register your number and if the person you're calling also registers, you can call each other free, if you initiate the call at the Jajah website. There is a lot of fine print on "free" calls and perhaps this is why the service isn't well known for this feature - but supposedly you can do it.
PhoneGnome is a very inconspicuous service that works like Jajah above, but the service appears to focus on free calls between members more than paid calls (which are almost hidden on the website). People can register their numbers and then call each other free. The service is free for both mobile phones and landlines in the US and Canada, but landlines-only outside the US. Other numbers can use the service, but they must pay a per minute fee to place calls.
Both PhoneGnome and Jajah have somewhat complicated pricing schemes for paid calls, that is, calls made from or to numbers that are not free (for whatever reason).
Both companies also have so-called "Fair Use" policies that put restrictions on the volume of free calls - but this is reasonable given that these companies are giving us something for nothing.
Jajah says the following:
JAJAH asks its customers to pay from time to time. If you choose not to pay, JAJAH may need to limit your free minutes. To take advantage of free minutes, users should deposit funds into their JAJAH account.
Again, I'd say that's fair. A lot of people complain that this kind of thing is a "rip-off", but I disagree. When they start playing bait and switch, that's one thing, but I think placing reasonable limits is perfectly fine, as long as they are transparent about it. You are getting a free service after all.
One thing I don't think is reasonable is the practice of random charges out of the blue, without warning, which has been reported about some of the services like VoIPcheap, VoIPbuster etc.
Jaxtr is another similar service, except instead of the service calling your phone when you place a call and then connecting you to the called party (which is how the above two work), with Jaxtr, the people that want to call you get a local telephone number to call you on. Instead of phone numbers, you know "friends" on Jaxtr by their email address. Jaxtr gives you a number for each "friend" (that is supposed to be local to you). When you call one of these numbers, it rings to that friends real phone (but you never need to give out your actual number).
Jaxtr limits users to a certain number of free calling credits each month and this seems to vary with the kind of account one has (they call them "jax") And, again, there is a complex scheme for the actual call costs (reminds me of "message units"). At this time, Jaxtr is still in Beta, and doesn't offer a way to buy additional credits.
Another important note. All this free calling generally applies to the same set of countries, or more directly, it always excludes places where there are few low-cost calling options, such as India, Africa, and the Middle East. There are VoIP options, including those listed above, that might be able to save some money for calls to/from these regions, but there are no free options. (If you're willing to buy a $99 hardware box, you can use the PhoneGnome service free in any of these regions).
In conclusion, as we state above, there really is no such thing as completely free VoIP, but there are ways to make zero-cost calls and significantly reduce your phone bills using a combination of free and low-cost calling options. This means that you will need to put some time and research into it, but in the end you can achieve almost free calls using VoIP technology and services.