Interlingua
There are two constructed languages which go by the name of Interlingua. One, Interlingua de Peano, is now more commonly known as Latino sine Flexione; the other, Interlingua de IALA, is the subject of this article.
Where the Esperanto vocabulary was drawn from Romance, Germanic, and Slavic languages and others, the vocabulary of Interlingua is drawn solely from Romance languages. Where the grammar of Esperanto was designed, and contains grammatical elements from wildly disparate languages, the grammar of Interlingua is based solely on the grammar of Romance languages, with some simplification. Interlingua was intended to be, in essence, the "average" of all the existing Romance languages.
The grammar and vocabulary of Interlingua were complete by 1950. Alexander Gode was one of the prime movers in this effort. He published a survey of the grammar, a one-way dictionary (Interlingua to English), and an introductory book entitled Interlingua a Prime Vista ("Interlingua at First Sight").
Interlingua still has some enthusiastic supporters in the United States, the Netherlands, Russia, and particularly Sweden. There are some Interlingua web pages, and at least one monthly newsletter.
Reviews and personal experiences: Anarres.
Interlingua sample: Our Father
Nostre Patre, qui es in le celos,
que tu nomine sia sanctificate;
que tu regno veni;
que tu voluntate sia facite
super le terra como etiam in le celo.
Da nos hodie nostre pan quotidian,
e pardona a nos nostre debitas
como nos pardona a nostre debitores,
e non duce nos in tentation,
sed libera nos de malo.