Essential to my recent decision to pack it all up and live on the road was the ability to maintain connectivity with friends, family, and work. In the past, I had to give the most lasting hugs I could because leaving the country rendered communication difficult, expensive, or impossible. Luckily, we live in a time where technology is catching up to the way we want to live, and I found a great solution to this problem by switching my number to Google Voice. Now I can send/receive calls and texts (almost) as if I never left. Here are some of the features that have been useful for me as a traveler.
I want access to my real phone number from anywhere in the world.
Here's the challenge. I have a cell number that I've been using for 10 years. I consider this to be my real number, and don't want to update all of my contacts every time I move around. Google Voice largely solves this problem. Upon porting my number, it lives in Googleland and I can seamlessly use my real number anywhere in the world through a computer or phone app via wi-fi.
But what about phone companies?
A key concept is that Google Voice is not a phone company, rather it is a wrapper around cellular service. This is confusing, but cool. When I need cell service in a new place, I pop in a sim card, get a temporary local phone number, and buy a bundle of minutes. The magic of Google Voice is that it takes all of these temporary phone numbers under its umbrella and makes them feel like my real number.
In other words, Google Voice is like a receptionist. When someone calls your number, Google Voice guides them to you, wherever you are, whatever local number you're using. It's beautiful and I'm able to totally avoid cell companies' bulky and overpriced international plans.
Also, thanks to the rise of pay-as-you go plans in the US, I only pay for cell service where I need it rather than paying down a useless 2 year contract while I'm out of the country. Google Voice allows me to seamlessly switch between my various types of short-term cell service.
It's not a perfect solution.
Despite the fun receptionist image, Google Voice does not solve 100% of travel connectivity issues. It doesn't work perfectly when outside of the US and outside of wifi range since Google Voice will not forward calls to my Argentine (for example) cellular service. To get around this I buy a local data package and use Google Voice through the data connection. This has worked reasonably well, though not perfectly.
Another downside is that Google Voice does not do MMS. So, when my sister-in-law sends a cute pic of my new niece, I miss out. This is kind of a bummer, but I try to remind friends and family to send pics via email instead.
Finally, as anyone who has ported a number from one carrier to another knows, this process sometimes takes several days to complete. I was not able to get all calls for a day or two and I couldn't get texts for about a week. So be careful to avoid the porting process during times when communication by cell is absolutely necessary.
Links I found helpful:
Groove IP Android App
Porting FAQ
About Google Voice
Pay-as-you-go in Buenos Aires
(Image: Flickr member khouri licensed for use under Creative Commons License)

Shaw's Original Fir...
I love Google Voice! I've been using it for years now. It doesn't work everywhere as you said. But! The the missed texts and voice mail should be waiting for you in "Googleland" when you're able to connect again.
Are you on iPhone/iPad? Use GV Mobile+ and Talkatone. Better than the official Google Voice app.
@Trishdom, Settings\Calls\Call Screening\Off. Turning it off messes with how GV and your cell phone (or any other phone connected to the number) handles voicemail, though.
I love Google Voice too. One big perk is being able to mark a phone number (ie for a telemarketer) as spam, so you never get calls from that number again.
Big fan of Google Voice and Talkatone
On balance, I like the call screening feature. yes, it's a pain to have to press 1 to accept the call but it's great to be able to press 2 and have it go to voice mail.
I also love the feature of being able to add a number to my contacts and always send those calls to voice mail without me having to do anything.
It's also great to be able to select times when the phone won't ring, if I'm working on something and don't want to be interrupted. Or, need a goodnight's sleep.
The translations can sometimes make my day. Often they're that 5-10% off which turn an ordinary message into something fun.
Yep. Another +1 for Google Voice and Talkatone on the iOS. It simply is unbelievable - my friends have no idea if I am out of the country, since their call, voicemail and SMS still gets through. Not that many of them have any ideas what it's like to be outside the US. :-p
The close runnerup is Skype, but it's not as free and it's not the same phone number.
I recently ported my Father's landline to Google Voice, purchased a OBi110 ($43.99 for the Obi100, or $49.99 for the Obi110 on Amazon) which allowed us to connect his home phones to GV, and now he gets free calls to the US an Canada... and the added convince of being able to call back to the US via his ipad (on wifi) when he is abroad, not to mention us being able to reach him without having to pay intl long distance charges...
here is a link to the forum post/guide that i followed... might help some others as well...
http://www.obitalk.com/forum/index.php?lgg=d7fpi4tddvk223ldhsg87vml25&topic=1051.0
I love google voice and use it when clients ask for my cell phone number. I can forward calls to my cell phone when I'm on the clock and turn it off afterwards. I especially love getting voicemails through my email when I'm out of the country so I can listen without phone charges.