Apache upload progress module. Works with mod_passenger (aka mod_rails). It's MIT license.

More info here: http://drogomir.com/blog/2008/6/18/upload-progress-bar-with-mod_passenger-and-apache

Tutorials showing how to use apache upload progress module:
http://www.drogomir.com/blog/2008/7/3/tweaking-rails-app-with-jquery-part-i (using jQuery)
http://www.railsillustrated.com/screencast-file-uploads-progress-in-rails-passenger.html (tutorial with screencast using prototype)

To install and activate module:
apxs2 -c -i -a mod_upload_progress.c

-c -> compile
-i -> install (copy upload_progress_module.so to apache lib folder)
-a -> activate (add LoadModule line to httpd.conf)

To compile it on mac you may need to:
sudo apxs -c -i -Wc,-arch -Wc,ppc7400 -Wl,-arch -Wl,ppc7400 -Wc,-arch -Wc,ppc64 -Wl,-arch -Wl,ppc64 -Wc,-arch -Wc,x86_64 -Wl,-arch -Wl,x86_64 -Wc,-arch -Wc,i386 -Wl,-arch -Wl,i386 mod_upload_progress.c

Global config:
  you can define maximum size for shared memory (default is 50kB):
  UploadProgressSharedMemorySize 1024000 # sets memory size to 1MB

Note:
If you get "(12)Cannot allocate memory: Upload Progress cache: could not create shared memory segment Configuration Failed" 
error you should try to set memory size to smaller value.

Config for vhost:
<Location />
    # enable tracking uploads in /
    TrackUploads On
</Location>

<Location /progress>
    # enable upload progress reports in /progress
    ReportUploads On
</Location>

- JSON-P Support
You can also request progress updates by using JSON-P, if you are uploading the file from a different domain or subdomain than the web server that is handling your original request. Adding a "callback=yourCallbackFunction" parameter to your request to the progress server will activate this functionality. 

For example, a request like:
  http://uploads.yourdomain.com/progress?callback=jsonp123&X-Progress-ID=1234
  
Would return the JSON-P function: 
  jsonp123(new Object({ 'state' : 'uploading', 'received' : 35587, 'size' : 716595, 'speed' : 35587 }));
  
The normal JSON request:
  http://www.yourdomain.com/progress?X-Progress-ID=1234

Would return the JSON data: 
  new Object({ 'state' : 'uploading', 'received' : 35587, 'size' : 716595, 'speed' : 35587 })