I'm disillusioned with Nike+, where I've been logging runs for over a year, and ready to try something new (in this case, Strava). How can I export my data from Nike+ so I'm not losing it all?
Endomondo.com works very well importing all the information from Nike+. After that you can use it to export individually to some other services, or use it as I think is pretty good.
Sorry folks for taking so long to get around to this update - I had to retire this tool in 2016 ago due to nike changing their API, and not having enough time to re-engineer the code, so this is no longer an acceptable solution.
I wrote a tool to help export workouts from nike+ for upload into other tools like Strava or Runkeeper: http://nikeplusexporter.rhysmccaig.com/
Trailrunnerx on osx can sync with nike+, including after Nike's recent changes to their API. From there one can export to various file formats. The gpx-tracks has worked for me for uploading to Strava and MapMyRun. Runkeeper on the other hand seemed to have trouble extracting the relevant data from these files. The Smashrun website can sync with nike+ directly. It's good for numbers, but doesn't have maps yet. MayMyRun also connects directly to Nike+, but doesn't seem to import gps data/maps and doesn't offer any graphs or statistics. It does so only when importing from files rather than through nike+.
Note that I've changed this answer several times, to keep up with current status.
In the previous months/years Nike has been changing the way it provides data; including who it provides it to.
I'd been using the exporter by Rhys McCaig for quite some time, but he has since discontinued his tool. I've also tried Tapiriik, which is great, but the Nike+ service is still in beta (I always get the message "Some activities could not be synchronized").
http://www.rungap.com/ supports a whole bunch of apps including Runkeeper, Garmin and Nike+. Unfortunately, it's doesn't provide online access. Just smartphone app access.
Update: Smashrun can import runs from Nike+ and Garmin directly. It also supports importing gpx/tcx files. I have the free membership for now. It is pretty bare bones, but from the looks of it the pro membership provides more valuable information and analytics.
The Android app SyncMyTracks can import and export from and to Nike+, Strava, RunKeeper, Endomondo, Garmin Connect and other services. I've been using it for a few months, and it works great!
I'm not sure what format the Nike files are, but the Fit File Repair Tool offers the most comprehensive options for conversion from and into the most commonly used formats (fit, tcx, and hrm). On top of that, it lets you fix faulty files, e.g. when the gps or the heart rate monitor went crazy due to interferences.
If conversion to tcx is sufficient, I found tcx creator to work quite smoothely. It's main purpose is to combine hrm files (from your heart rate monitor) with gpx files (from your gps device), but it also works if you skip the gpx files and just feed it with hrm.
So basically, what you need to do is check what file format Nike produced for you and check what your new online service can import (most likely one or more of the above). According to the tcx creator website, all of the following accept tcx files: endomondo.com, strava.com, connect.garmin.com, runkeeper.com
The (free) online app called Tapiriik provides a great way to sync between many applications. Including endomondo, strave, runkeeper, dropbox, garmin, ride with gps, motivato, and many others.
If you have an iOS device, you might find this app handy: https://rungap.zendesk.com/entries/45048583-Using-Endomondo-with-RunGap
I have written as well a PHP script to export those data: https://github.com/cheesemacfly/nikeplusexport
(there is a link to a demo on the page)
The main difference I have with the other ones is that the time for each waypoint is calculated so I found it more precise.
It works only with the runs having GPS data and can export to GPX, TCX and JSON.