If you want to host several development projects at once, but are tired of creating hostnames and directories for every single project, use a setup like this. It uses mod_vhost_alias and requires a wildcard DNS entry, like *.dev.example.com for example. For completeness, create dev.example.com too.
Redirect dev.example.com to www.dev.example.com. That will put it in the catch-all vhostalias virtualhost:
<VirtualHost *:80> ServerName dev.example.com Redirect permanent / http://www.dev.example.com/ </VirtualHost>
A virtualhost needs a valid server name, so we put that here. Include the wildcard here, too.
<VirtualHost *:80>
ServerName www.dev.example.com
ServerAlias *.dev.example.com
# %-4+ gives you the 4th part of the hostname from the end, and everything preceding that.
# www.dev.example.com becomes www
# sub.domain.dev.example.com becomes sub.domain
VirtualDocumentRoot /www/dev.example.com/%-4+/htdocs/
CustomLog /var/log/apache2/dev.example.com-access.log combined
ErrorLog /var/log/apache2/dev.example.com-error.log
# like above, %-4+ gives you the 4th part of the hostname from the end, plus everything before that.
# this will setup a /cgi-bin/ ScriptAlias for every vhostalias.
VirtualScriptAlias /www/dev.example.com/%-4+/cgi-bin/
# specify options and overrides here.
<Directory /www/dev.example.com/>
Options None
AllowOverride None
Order deny,allow
</Directory>
</VirtualHost>
Now, all you need to do is create /www/dev.example.com/www/htdocs/ and /www/dev.example.com/www/cgi-bin/ to get started. From then on, whenever you need a new hostname, just create the appropriate directories.