One of Tchaikovsky's unjustly neglected orchestral suites features two accordians!
Vaughn Williams wrote a "Romance" for harmonica and orchestra. His bass tuba concerto is a rarity too.
Mahler's 6th symphony uses a hammer of some sort. Guitars (and mandolins?) appear in his 7th, and his 8th features mandolins, a harmonium, a piano, an organ, and probably other unusual instruments.
R. Strauss used a wind machine in Don Quixote and Ein Alpensimphonie (sp?). I think Ravel used one in Daphnis et Chloe, and Vaughn Williams used one in at least one symphony.
Nielsen used a male and female vocalist, singing wordless vowel sounds, in a movement of one of his symphonies (sorry, forgot which one).
I don't know if Verdi used actual anvils in his Anvil Chorus, but they don't sound like any standard percussion instrument, whatever they are.
R. Strauss's Till Eulenspiegels Lustige Streiche and Symphonia Domestica, and Respighi's Feste Romane, seem to contain some unusual percussion instruments.