A lot of essential web tools in one place:
Protolize

Perhaps one of the hardest things to master... Regular expressions.

But there's hope: a website dedicated to this cryptical yet powerful phenomenon:
RexExpLib.

Need a temporary, disposable email address?

Check this out:
38 resources on disposable email services

Some (very nice) CSS related websites:

CSS Various:

• CSS Play by Stu Nicholls
• CSS Zen Garden
• Web Reference
  

CSS Galleries:

• CSS Import
• CSS Mania
• CSS Vault
• Unmatched Style

Check this out:
http://www.connectionstrings.com/

Nearly every connection string you can think of...

(Thanks to Remon)