Get the current page URL (including pagination)

In addition to Rajeev Vyas’s answer, you don’t need to pass any non-empty parameters to add_query_arg(). The following has always worked well for me:

// relative current URI:
$current_rel_uri = add_query_arg( NULL, NULL );

// absolute current URI (on single site):
$current_uri = home_url( add_query_arg( NULL, NULL ) );

The function falls back on $_SERVER[ 'REQUEST_URI' ] and applies urlencode_deep() to it. See https://github.com/WordPress/WordPress/blob/3.8/wp-includes/functions.php#L673

Edit:
As $_SERVER[ 'REQUEST_URI' ] represents unfiltered user input, one should always escape the return value of add_query_arg() when the context is changed. For example, use esc_url_raw() for DB usage or esc_attr() or esc_url() for HTML.

Update

The shown example that should create an absolute URI (containing scheme and host) does not work on multisite with sub-directories as home_url() would return the complete URI including a path segment. A better solution for multisite aware code would be this:

// absolute URI in multisite aware environment
$parts = parse_url( home_url() );
$current_uri = "{$parts['scheme']}://{$parts['host']}" . add_query_arg( NULL, NULL );

WordPress core does not support port, user or password in a multisite site URL so this should be sufficient.