Thought I'd contribute something and make a list of the useful things Google provides

Hopefully we can build this list up into a neat reference.

Operator “site”: eg site: http://www.example.com searchQuery - returns list of pages in a site with the term 'searchQuery'

Operator “intitle”: eg intitle:snuffles hack - results with snuffles in the title and hack in the text

Operator “allintitle”: just extends the above. Eg. allintitle:kingdom come - results with 'kingdom come' in title

Operator “inurl”: Much the same format as intitle but searchs for URLS instead.

Operator “allinurl”: Same as allintitle except for URL's

Operator “filetype”: Looking for a filetype? This is your query! EG filetype:txt tomorrow - returns all txt files with 'tomorrow' in the file.

Operator “numrange”: Restricts results with the numrange in them. EG. numrange:1-10 or numrange:1..10 returns all pages with numbers 1 to 10 in them.

Operator “link”: Returns links found by Google to a URL. Eg. link: http://mysite.com

Operator “inanchor”: returns results from links in pages. EG inanchor: myLife - returns all pages with 'myLife' as the text for links.

Operator “allintext”: - just searches text. Not title or URL,.

Operator “+”: - returns pages in order of frequency of the term.

Operator “-”: returns pages WITHOUT the text

Operator “”: returns pages with all words in phrase. EG "my life sucks" - returns pages with 'my life sucks' (all words) in the text.

Operator “.”: Wildcard character

Operator “|”: Logical operator - OR. EG John|Joe - returns either John OR Joe.

Good stuff there.

Here's a few more to get people started:

intitle:"Index of" .mysql_history - shows up mysql history files
intitle:"Index of" pwd.db - yes, passwords which will most probably be hashed, but not unbreakable!
filetype:htpasswd htpasswd - yup, some people are stupid or silly enough to leave htpasswd files lying around. Even if the passwords no longer work, the webmaster may just be using them on another site!
intitle:index.of config.php Why anyone would have a config file indexed by google is beyond me.