I used to do this, except that if one of those sites ever leaks the passwords plaintext, you re screwed everywhere, since it'll be trivial to add 'ReallyHardPassword' as a prefix for the site. Ditto if you cleverly change google->G0o9Le.
As for 'salting', most* people use some variation of uppercase first letter, uppercase last and/or leding/ending "1" as their password. (Clever ones use 0 instead of 1) so if your password is "1ReallyHardPasswordGoogle!1" well, congratultions, you can now open my luggage ;)
As many people point out, you really should have unique passwords for every site. For the really paranoid, you could use your password manager's password as salt, provided you trust their rnd genertor (spoiler: you must). It's trivial to just -manually- add the site name to the password form, so if you want to log in as me to slashdot, you'd use "aeCxXAk&+5a_s1&&Slashdot"