Novial was developed by Otto Jespersen and first published in 1928. It has undergone a few revisions, including the current Novial98 project.
Esperanto was developed by Ludovik ZAMENHOF, and published around 1887. It is probably the most widely known constructed language, claiming somewhere from 50 000 to 2 000 000 speakers.
Ido was itself a reform of Esperanto, developed around 1907 by Louis Couturat and others. It mostly died down after a few years, but may be making a comeback.
Interlingua was developed in the 1950s by the IALA. It tries for a more "naturalistic" feel, and thus looks much like a Romance language.
Occidental (aka "Interlingue") was a very naturalistic auxlang developed by Edgar de Wahl in the mid-1920s. It was contemporary with Novial, and showed many similar features (and indeed later Novial displayed many reforms to bring it closer to Occidental). When Interlingua was invented, almost all Occidentalists appear to have gone over to it.
Lingua Franca was the IAL to know in the Mediterranean, lasting from the Middle Ages until the nineteenth century.
Eurolang is a modern IAL designed by Phil Hunt.
Glosa was created by Ron Clark and Wendy Ashby, based on Hogben's Interglossa, based on Latin and Greek; it is highly isolating.
Volapük was the first IAL to achieve any real success; it was invented in 1879 by Joseph Schleyer and revised in 1931 by Arie de Jong. It is highly inflected, and while word roots were pulled from natlangs, they were usually modified so much as to be unrecognizable.
Dutton Speedwords (aka "Rap Lin Rie") were designed in the 1950s as a sort of shorthand written language appropriate to multiple spoken languages.
Folkspraak was a collaboratively designed inter-Germanic auxlang; it seems to be dormant since mid-1996.
Frater 2, by Paul Bartlett, based on Frater, which was developed in Vietnam in 1957.
Big Six (formerly "P", "Pig") is a conIAL by Danny Wier based on the six most spoken languages: Mandarin, Hindi, Spanish, English, Russian, and Arabic.
Latin sine Flexione is Giuseppe Peano's attempt; he took the vocabulary of Latin and removed the inflexion, leaving the part-of-speech marking and tense marking and such, but removing agreement, thus eliminating all those nasty declension and conjugation tables.
Tok Pisin is a pidgin on the border of being a creole which a few members of the AUXLANG list think would make a very good IAL.
Lingua Franca Nova is by George Boeree, based primarily on the Romance languages (apparently very heavily on Italian and Spanish).
Poliespo is (supposedly) a combination of Cherokee and Esperanto. Totally bizarre. Perhaps this should be on the conlang page.