Yes it is enabled by default but make sure your framework or app httpd-app.conf has AllowOverride set to All.
<directory "..."> ... AllowOverride All ... </directory>
For Codeigniter, make sure your .htaccess is in the same dir as your root index.php.
RewriteEngine on RewriteCond $1 !^(index\.php|images|robots\.txt) RewriteRule ^(.*)$ /index.php/$1 [L]