This is the US companion of the same question for Canadians Where can I park my rainy-day/ emergency fund?. Basically I want to know what options I have for my emergency fund?
First off, you generally want to park your emergency fund somewhere that is "safe", meaning something that is not subject to market fluctuations. Your emergency fund is something you need to be able to count on when times are tough! That rules out things like stock market investments.
Secondly, you need to think about how quickly you will need access to the money. If you have an emergency, odds are you don't want to be waiting around for weeks/months/years for the money to become available. This rules out most fixed-term investments (Bonds, traditional CDs, etc).
If you are concerned that you will need near-instant access to your emergency money, then you probably want to keep it in a Savings or Money Market Account at the same bank as your checking account. Most banks will let you transfer money between local accounts instantly.
Unfortunately, your local bank probably has pitiful interest rates for the Savings/MMA, far below the inflation rate. This means your money will slowly lose value over time. Be prepared to keep contributing to it!
For most people, being able to draw the cash from your fund within a few days (<1 week) is sufficient. Worst case, you charge something on your credit card, and then pay down the card when the emergency fund withdrawal arrives.
If "money within a few days" is okay for you, there are a few options:
Money Market (Mutual) Funds (not to be confused with a Money Market Account) - This is the traditional place to keep an emergency fund. These are investment funds you can buy with a brokerage account. An example of such a fund would be Fidelity Cash Reserves. MMFs are not FDIC insured, so they are not exactly zero risk. However, they are considered extremely safe. They almost never go down in value (only a few times in the past few decades), and when they have, the fund manager or the Federal Govt stepped in to restore the value. They usually offer slightly better return than a local savings account, and are available in taxable and non-taxable varieties.
Online High-Yield Savings or Money Market Account - These are a relatively new invention. It's basically a the same thing as what your local bank offers, but it's online-only. No local branch means low overhead, so they offer higher interest rates (2.0% vs 0.5% for your local bank). Some of them used to be over 5% before the economy tanked. Like your local bank, it is FDIC insured.
One bit of caution: Some of these accounts have become "gimmicky" lately. They have started to do things like promo rates for a few months, only offering the high interest rate on the first few $K deposited, limiting the amount that can be withdrawn, etc. Be sure to read the details before you open an account!
No-Penalty CDs - Certificates of Deposit usually offer a better rate than a Savings Account, but your money is locked up until the CD term is up (e.g. 36 months). If you need to cash out before then, you pay a penalty. Some banks have begun to offer CDs that you can cash out with no penalty at all. These can offer better rates than the savings account. Make sure it really is no-penalty though. Also watch what your options are for slowly adding money over time. This can be an issue if you want to deposit $100 from every paycheck.
Rewards Checking Accounts - These are checking accounts that will pay a relatively high interest rate (3% or more) provided you generate enough activity. Most of them will have requirements like you must have direct deposit setup with them, and you must do a minimum number of debit card transactions from the account per month. If you can stay on top of the requirements, these can be a great deal. If you don't stay on top of it, your interest rate usually drops back to something pitiful, though.
Personally, we use the Online High-Yield Savings Account for our emergency fund. I'm not going to make a specific recommendation as to which bank to use. The best deal changes almost week to week. Instead, I will say to check out Bankrate.com for a list of savings accounts and CDs that you can sort. The Bank Deals blog is a good place to follow rate changes.
I would suggest your local credit union or local bank for security and liquidity. Liquidity is probably the most important issue for a emergency fund.
I am using ING for my emergency savings, but sometime last year I discovered SmartyPig. As of 4/24/2010 they offer 2.1%, which is even better than the 1 year CDs at most banks. I've switched two small accounts to SmartyPig and plan to switch my emergency savings. Their accounts are geared around monthly contributions, but you don't have to use that feature.
Something with an FDIC guarentee, so a bank. With an emergency fund, I think the 'return of capital' is more important than the 'return on the capital', so I'm fine with putting it in a standard savings account in a local bank(not an internet account) even if it pays next to nothing. The beauty is that since the bank is local, you can walk in and withdraw it all during any weekday.