Was doing this on multiple servers today, might as well get a blog entry out of it.
Note: this requires mod_so to be installed.. w/o that you'll need to do a lot more work.
To find out if you have mod_so, use /usr/local/apache/bin/httpd -l

1. Download mod_throttle
   http://www.snert.com/Software/mod_throttle/mod_throttle312.tgz


2. upack & unzip the tgz file
   tar -xzvf mod_throttle312.tgz


3. Edit mod_throttle.c Change: (this is for a freebsd system)
   
   #define USE_POSIX_SERIALIZATION
   #undef USE_SYSTEM_V_SERIALIZATION


4. Edit Makefile
   
   Important stuff:
   1. APXS=/usr/local/apache/bin/apxs
   2. APACHECTL=/usr/local/apache/bin/apachectl


5. make install (that's not a mistake, don't need to do make first).
   You'll need to do this as root or using sudo since it edits the httpd.conf file.


6. Edit httpd.conf to set a throttle policy
   
   #
   # Throttle policy limits connections to 100k/s per IP address
   # Keeps a history of 1000 ip addresses
   #
   
   <IfModule mod_throttle.c>
      ThrottleClientIP 1000
      ThrottlePolicy Speed 100K 1s
      <Location /ts>
         SetHandler throttle-status
      </Location>
   </IfModule>
   


7. Restart apache
   
   /usr/local/apache/bin/apachectl restart


8. You're now throttling the entire server to 100k/s per IP.