## no-www redirect
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L] 

## www redirect
RewriteCond %{HTTP_HOST} ^example.com [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]

## multiple domain redirect
RewriteCond %{HTTP_HOST} !^example\.com$ [NC]
RewriteRule ^(.*)$ http://example.com/$1 [R=301,L]


## trailing slash
## BE EXTRA CAREFUL WITH THIS ONE!
## POST REQUESTS WITHOUT TRAILING SLASH
## WILL BE IGNORED!
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ http://www.example.com/$1/ [L,R=301]

## no-www redirect
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L] 

## www redirect
RewriteCond %{HTTP_HOST} ^example.com [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]

## trailing slash
## BE EXTRA CAREFUL WITH THIS ONE!
## POST REQUESTS WITHOUT TRAILING SLASH
## WILL BE IGNORED!
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ http://www.example.com/$1/ [L,R=301]

## no-www redirect
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L] 

## www redirect
RewriteCond %{HTTP_HOST} ^example.com [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]

## trailing slash
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ http://www.example.com/$1/ [L,R=301]

Please, be careful with trailing slash rewrite as post requests without a trailing slash will be ignored.

Be sure to test if you plan on using that one.
Not compatible with Drupal.

Common .htaccess Rewrite Rules

htaccess

Apache