At first glance, Gizmo Project seems like Skype. However, there are differences, some mentioned here before. One is an invisble feature, namely that Gizmo is SIP-based and therefore open sourced. So not only can you call other Gizmo members (PC, PSTN) but also members of other SIP-based networks. In fact, you can now at least IM Google Talk users, with actual softVoIP calling to come.
With the recent announcement, you can also call to PSTN phones in 60 countries for free - a potential market of 2 billion phones - provided the person you're calling is a registered member of Gizmo. If they're not, Gizmo CallOut (similar to SkypeOut) offers low per-minute rates. The free calling to 60 countries is a permanent feature, not a promo. (There is a noticeable absence of China and India on this list.)
Gizmo Call In gives you a free US-, UK-, France- or Spain-based phone number for US$3/m, payable in 3- or 12-month increments. Interesting that Canada is not part of that, especially considering Skype's 2006 promo for free calls between the US and Canada. (I.e., there's a VoIP market in Canada.) Over all, you can have a phone number from over 50 cities. (How cool would it be to have, say, an L.A. phone number yet live on the East Coast. East meets West.)
Essentially it means that if you live in one of the cities represented, you can travel and still receive calls as if you're local. Less cost for your friends. It's also ideal for people who've moved - or relocated temporarily - and want friends and family to be able to contact them without long distance charges.
One other call option is an US-based Area775 number, although in the same breath it's described as being both free and costing a small monthly fee. (Don't know how that's possible.) When other people dial your Area775 number, it can call both your computer or regular phone. Calls can be screened, transferred, or shuttled to voicemail. The latter has the option of generating SMS messages. (Having your regular phone called costs $2/call.)
For those of you that get calls from strangers - like I occasionally do - Gizmo Project has a Google map that shows you their call location. This is ideal for for flagging potential Vishers who tell you that they're from somewhere else.
If you have need of conference calling, Gizmo Project has FreeConferenceCallTM, which allows calls between landlines, cell phones, and Gizmo softVoIP users. Gizmo users initiate the call with a free conferencing number. Other phones have to dial a (non-toll-free) number, then the conference room #. Of course, if other callers are using Gizmo, there's no long distance cost.
There's also support for Asterisk PBX, an open source PBX designed for enterprise VoIP, and a host of other features. Or if you want to make outbound calls from a PSTN phone, you can buy Gizmo-compatible SIP adapters.
Finally, if you're a developer and want to build VoIP applications, they have an API (Application Programmer Interface) and SDK (Software Development Kit) for both Windows and Mac OS X, with a Linux SDK coming soon. The SDKs are unfortunately only in C++, but a lot of Java and other object-oriented programmers do not have too much trouble with C++.
At the most basic level, the API and SDK allow you to create your own branded SIP-based softVoIP phone, so businesses could have a soft client with their logo.