An OTA used in synth circuits to make VCAs and VCFs. Full designation is LM13700; it was originally designed by National Semiconductor in the early 1970s, to compete with RCA's recently introduced CA3080. (Legend has it that it was designed by an intern as a layout exercise.) Although it is considered to be a somewhat superior design to the CA3080 due to the presence of linearising input diodes, the LM13700 was not quite as popular in synth designs in the heyday of analog synthesis in the '70s and '80s.
As of late 2011, the LM13700 is the only OTA integrated circuit currently being manufactured; it is available from the usual online outlets for about $1 US, quantity 1. (There was a variant, designated the LM13600, which was introduced at the same time; it went out of production in 2008.) It is still made by National and several other sources.