I've always been a fan of Drow sign, personally. In a game I ran that had a paladin who'd taken a vow of silence, I house ruled in Common Sign. He & the half-drow ended up cobbling together a language of their own that was a mix between the two. That was fun. Especially when he started making signs to denote the various party members. The winged elf turned into Wings, the half-drow who traveled around on a Floating Disc became Disc, the wood elf became Copper for her skin...I think the halfling was just something like "Short." :P