Ray Monk in Prospect:
His essay “On Sense and Reference” (1892) offered a philosophical account of linguistic meaning that broke new ground in sophistication and rigour, and it is still required reading for anyone who wants to understand contemporary philosophy of language. It is scarcely an exaggeration to say that he invented modern logic: he developed the basic ideas (if not the symbols now in use) of predicate logic, considered by most analytic philosophers to be an essential tool of their trade and a required part of almost every philosophy undergraduate degree programme. His book The Foundations of Arithmetic (1884) is still hailed as a paradigm of the kind of crisp, rigorous prose to which every analytic philosopher should aspire.
Frege’s insights have been influential outside philosophy, in areas including cognitive science, linguistics and computer science. Among the public, however, he is almost completely unknown, especially when put beside Wittgenstein and Russell. Most people have some idea who Russell was. Many have seen clips of his frequent appearances on television, can picture his bird-like features crowned with his mane of white hair, and recognise his unique voice, high-pitched, precise and aristocratic in an impossibly old-fashioned way (one imagines that no-one has spoken like that since the Regency). Even better known is Wittgenstein, the subject of a Derek Jarman movie and several poems, whose name is dropped by journalists, novelists and playwrights, confident that their audiences will have some idea who he is. But Frege? How many people know anything about him?
More here.