$ kill -USR1 `cat /path/to/httpd.pid`
[download]

after you've made changes to modules will cause apache to finish serving requests in progress and reload (rereads config files also). or use StatINC as others have mentioned to have it done automagically

my understanding is that changes to scripts served up like plain old CGI will be noticed automatically, the server does the normal stat of the file and reload it if it's been modified. for stuff modules StatINC forces the same behavior.