Go to Linux Advanced Routing and Traffic Control
. Slackware probably has the proper commands ('ip' and 'tc'). If not, look for an iproute2 package. This is explained in the LARTC HOWTO. Specifically, look at this page
for rate limiting. You probably should read most of the HOWTO so you can understand what's going on. You may also want to look at the "traffic conditioner" wondershaper
, which tries to keep interactive traffic fast, even when your upload is maxed out. Look at this page
for an explaination of what it tries to do.
It's a bit more low level than a squid solution and probably takes a bit more knowledge, but when you get it set up it can improve perfomance (especially for online games). It also limits all traffic (read Kazaa, gnutella, freenet and other bandwidth wasters), not just HTTP.