mod-bandwidth for Apache 1.3 is widely referenced.

I struggled to find something similar for Apache 2. libapache2-mod-cband didn't seem to do the tricks we needed.

Several distro's already include mod-bw.

See http://apache.ivn.cl/

I've filed a RTP (481909), which includes a fix for compiling on Debian Apache version 2.0.x (read Sarge).

The merits of slowing down downloads, thus trading bandwidth for other resources is a different question, but if you want to try it you now know how.