Fidelity is a great source for looking up stocks, mutual funds, indexes and annuities. And, it's free!
Here's what to do when you are on the Fidelity site:
Set "Search for:" to the type of security you are looking for (Stock, Mutual Fund, Index or Annuity) and "by:" to the information you have to enter (Security Name, Trading Symbol, CUSIP Number, Fund Number) and "Search Value:" for the security description or symbol of what you are looking-up and click "Search." The site will respond with all the information it has on your selection.
QuantumOnline is another great source for running a quick search using ticker symbol, cusip number or name, and it's also free! It provides symbols and cusip numbers for stocks and mutual funds.
MSRB is a great source for looking up cusip numbers for municipal bonds using their Muni Search, and this tool is also free!
CUSIP Global Services is another good source. For a fee, you can access the entire universe of CUSIP numbers (Corporate, Municipal, Government, Mortgage-Backed and Private Placement) and ISIDPlus - the global cross-reference database.