monline

The two most popular solutions:

• trickle, a userspace bandwidth shaper. It's just this easy:
  
  trickle -d 200 -u 30 firefox
• Wonder Shaper, a kernelspace bandwidth shaper. More efficient for interface-level bandwidth shaping (but does not support per application control at all).

Before using trickle you should know that it works only if your app is dynamically linked and uses libc. You can check this with ldd. Eg. in case of Firefox in Ubuntu Karmic:

I should add though that tickle did not work for me with Firefox. I'm not sure why. However it worked with Epiphany. Upload speed limit was quite far from what I've set (it was off by 20-50%).