(FORTUNE Magazine) – CDs are hot again. Considered dull when the stock market looked as though it went only up, those humdrum old certificates of deposit are getting more attention from investors. They're also getting pushed a lot harder by banks, brokerage houses, and investment funds -- and cast in some snazzier forms. The upshot for investors is that the simple has become complex. ''Some banks are using gimmicks -- bells and whistles -- to get new CD business,'' says Robert Heady, publisher of Bank Rate Monitor, a newsletter in North Palm Beach, Florida. Brokers also have their share of whistles. Some use the product as a loss leader to attract other kinds of investment business. Investors can best circumvent all this confusion by focusing on one number: effective yield. That's the interest received including compounding -- some certificates are compounded daily, others at intervals as long as six months. Armed with actual yield, it's easier to sort out the opportunities provided by the hybrid CDs. One of them is the variable rate CD, often tied to the bank prime rate. As the prime rises, the CD goes up in tandem. That means an investor isn't left out in the cold when yields move higher, but the initial rate is generally lower than on other CDs and in some cases can fall as well as rise. Another hybrid is the stock index CD, which has a yield generally tied to that of the S&P 500. Intense competition has been pushing up rates. The Texas banks and S&Ls that offered the best deals before they fell on hard times have been replaced by financial institutions in the Northeast and California. California Professional Savings in Beverly Hills was recently paying 8.60% on a six-month CD, compared with the national average of 7.58%. Some banks in the Northeast are offering almost as much as the S&Ls. Make certain that the high-rate CD carries government insurance up to $100,000 per person, per account. Another caveat: Interest and principle are covered only up to the date of liquidation, which means that your CD stops accruing interest when an institution fails. Brokerage houses are pushing hard to invade bank and S&L turf. The result is that most brokered CDs have higher yields than the average of their banking counterparts. Reason: Brokers can choose from a network of banks around the nation. Prudential-Bache, which has one of the highest-yielding CDs (8.45%), culls a list of over a hundred banks to pull out some 25 issues. Its selection, like that of most brokers, changes from week to week. The only potential problem is that brokered CDs sometimes skimp on quality to increase ^ yield; one of Pru-Bache's issuing S&Ls closed its doors within the last 18 months. Brokers also have access to secondary markets, where CDs can be bought and sold much like bonds as interest rates fluctuate. ''Investors aren't locked up for three to five years at a specific rate because of the brokers' extensive system,'' says Peggy Hall, editor of Donoghue's Money Letter in Holliston, Massachusetts. Though customers generally pay no commission to the broker when purchasing new CDs, those purchased on the secondary market carry hidden charges in the form of a spread between bid and asked prices. Not to be outdone, mutual funds are competing hard for CD buyers. Dreyfus recently launched an innovative product called the Investor CD that can be withdrawn after a week without penalty and placed in a stock or bond fund. Its recent yield was 8.20%. And after the stock market crash last fall, Fidelity brought out its 24-hour CD Shopping Service, which now offers a choice of 15 to 30 issues on any given week. Recently its highest-yielding six-month CD was 8.35%.

CHART: NOT AVAILABLE CREDIT: NO CREDIT CAPTION: WATCHING THE RATES Investors are taking a longer look at CD offerings, such as this list at the Dreyfus Consumer Bank, East Orange, New Jersey. DESCRIPTION: Highest-yielding six-month certificates of deposit offered by S& Ls, retail brokers, commercial banks.