Image credit: Susie
I started out with Yahoo's Fire Eagle, but by itself it's not really useful. As with so many services, ease of use is again the differentiator. Manually updating your location if you are moving around is just too much of a hassle.
Next service to try was Dopplr... I love it.. I can keep track of my trips and can plan them for the future. The FriendFeed and Facebook integration nicely informs my friends. This already saved me once when my coworkers thought I disappeared, although I was just on a business trip. I look forward to my annual travel report.
The iPhone changed the ease of use for location based applications. My favorite Twitter application is Twinkle, because it's nice to find people near you. However I'm becoming more and more a fan of Brightkite. Especially the iPhone application is nice for making location based notes and photos. Brightkite also has nice Facebook and Twitter integration to update your friends of your whereabouts.
However, all these services are only cool if you go somewhere... Lets see what trips come up in 2009