While the emergence of VoIP, or voice-over-Internet protocol, technology has already helped push down the cost of making a phone call, now it’s starting to have a deflationary impact on the world of mobile, where call charges remain stubbornly high.

In the meantime, the ongoing adoption of 3G broadband and the inclusion of Wi-Fi in many high-end phones is drawing a growing amount of attention to mobile VoIP services. Indeed, research firm Disruptive Analysis predicts that the number of VoIP-over-3G users will top 250 million by the end of 2012 — from virtually zero in 2007.

We at GigaOM are constantly tinkering with these mobile services, so we’ve put together a list of seven mobile VoIP apps that we think you’ll find handy.

Skype Options

Service: Skype Mobile
Platform/Network: Java-based application that works on 50 popular phones from Nokia, Sony Ericsson, Motorola and Samsung. Can be used on numerous cellular data networks.
Cost: Free
Features: Chatting (including with a group), presence settings (offline, online, do not disturb), and Skype-to-Skype calls (including SkypeIn).
Our Opinion: If your phone is supported, Skype Mobile is a great way to add Skype chatting and calls to it. Though it would be nice if the Skype application weren’t written in Java, as these applications are often sluggish and unstable.

Service: 3 Skypephone
Platform/Network: Uses a specialized handset. Currently available in the UK, Italy, Austria, Hong Kong, Australia, Ireland, Denmark and Sweden.
Cost: The phone costs £49.99 (about $98) and can be used on a pre-paid basis. Calls cost nothing if they’re made from Skype.
Features: Free Skype-to-Skype mobile calls and the ability to conduct Skype IM conversations, all without touching your computer.
Our Opinion: The 3 Skypephone is best suited for those who need a few monthly mobile minutes and wish to talk/IM to their Skype friends along the way. The pre-paid feature makes it an attractive and inexpensive option for Skype chatters in the geographies it serves.

Service: iSkoot
Platform/Network: Mobile handsets such as BlackBerry, Nokia, Windows Mobile and Palm OS models. Also works on GSM networks.
Cost: Modest to expensive, based on usage. Because iSkoot is a hybrid VoIP/GSM service, it uses SMS and mobile minutes when making and receiving calls or Skype IM messages.
Features: SkypeIn and Skype-to-Skype calls, Skype IM messages.
Our Opinion: iSkoot is a good option for keeping in touch with your Skype contacts. However, I would look at other software applications that just use data to send Skype SMS and Skype voice traffic.

Mobile VoIP Players

Service: Truphone
Platform/Network: Nokia handsets
Cost: Incoming free calls while on the Truphone network, low per-minute rates while on a GSM network. Outgoing calls are billed at very low per minute rates.
Features: Truphone offers free calls, SMS and voice mail while logged into the Truphone network via Wi-Fi. Otherwise, Truphone forwards calls to your mobile handset and you pay a low per-minute charge.
Our Opinion: With its smart forwarding options, Truphone is particularly useful for international travelers. Whether you’re on Wi-Fi or just your normal GSM network, you can be reached via your Truphone number no matter where you are (charges apply in certain cases, see their site for details). For times when Wi-Fi is not available, Truphone just released Truphone Anywhere, which utilizes local gateways for outgoing calls at low per-minute charges. I have trialed the service by forwarding calls from my Truphone number to my cell phone and the call quality was fantastic; voices were indistinguishable from any other cell phone call. A Truphone-to-Truphone VoIP call yielded even higher voice quality.

Service: Fring
Platform/Network: Nokia/Symbian handsets, Widows Mobile, iPhone (pre-release beta)
Cost: Free
Features: Allows you to make VoIP calls on any SIP network, Skype or to other Fring users. Additionally, Fring is a multi-protocol IM client that will allow you to chat with your buddies on Skype, MSN, ICQ, Google Talk, Twitter, AIM and Yahoo.
Our Opinion: I have used Fring to make SIP and Skype voice calls, and over EDGE the call is choppy and hard to understand. However, Wi-Fi provides enough bandwidth to make Fring calls clear and understandable. I wouldn’t say the quality is fantastic, but it is very comparable to a normal cell phone call.

Service: Talkonaut
Platform/Network: Available for Java-based phones, Symbian and Windows Mobile
Cost: Free
Features: Talkonaut offers free VoIP and IM chatting. For instant messaging, the application supports Google Talk, ICQ, AIM, and Yahoo. Talkonaut can also use SIP for VoIP calling.
Our Opinion: Talkonaut is a Russian offering and is still very new. The application was quite unstable on my Nokia N82 handset. With iffy performance and a very rusty user interface, I would recommend looking at other applications for this functionality, namely Fring.

Service: Nimbuzz
Platform/Network: Java program, Symbian; an iPhone version is coming soon.
Cost: Free
Features: Allows you to engage in IM conversations and conduct VoIP calls, as well as to share media such as photos and video. Nimbuzz also allows for client-to-client calls and has widgets enabling calls to originate from Facebok and MySpace. Compatible with Skype, Google Talk, AOL Instant Messenger, Windows Live Messenger, Yahoo, MySpace, Facebook and Jabber IM networks.
Our Opinion: The Symbian application is very responsive and easy to use. A VoIP call originating from my Facebook page to Nimbuzz over a Wi-Fi connection sounded good — just as good as any call over a GSM network.

Service: Gizmo5 (formerly Gizmo Project)
Platform/Network: Nokia Symbian handsets, BlackBerry, Windows Mobile, Nokia Tablets
Cost: Free for VoIP calls, low per-minute charge to call landlines and to SMS to mobiles.
Features: Gizmo5 allows you to conduct voice calls to other Gizmo5 users and grants IM conversations with friends on Gizmo5, MSN, AIM, Yahoo and Jabber.
Our Opinion: Gizmo5 is a great competitor to Skype, just not as established or well-known. They have wisely developed their client software for many platforms including numerous mobile phones, and on both Mac and PC. VoIP call quality is stellar when calling between clients, as well as to landline/mobile phones.