The National Institute of Standards and Technology has recently completed selection of an algorithm for its Advanced Encryption Standard (AES). The winner was Rijndael which is thereby guaranteed to become a widely-used algorithm. Rijndael is both powerful and versatile, and makes a good choice for the AES selection, and for general use.

Counterpane's Blowfish has been popular for a number of years. Its successor, Twofish was another AES finalist that is likely to continue in widespread use (despite the selection of Rijndael as the winner).

The most widely-used symmetrical encryption algorithm has almost certainly been NIST's (formerly called National Bureau of Standards) Data Encryption Standard (DES). Although DES has developed key length problems with the advancement of computer capabilities, triple-DES is still viable, and even single-DES is an algorithm you are likely to come across in existing products.