SPRECTRUM SP-1:
In 1977 the SP-1 was released as part of the “traffic light series”. The SP-1 is an EQ. A parametric EQ, that boosts a specific range of frequencies. The SP-1 proved to be very popular with Keyboard and Bass players who wanted to stand out in a mix. With only 2 knobs the Spectrum knob selected a frequency and the Balance knob selected a point in that frequency to boost 500Hz – 5kHz.
Rarity: Rare
Usefulness: Low – Moderate
Price: $300 - $500
Cheaper Alternative: Any Parametric EQ
Example: 
Slow Gear SG-1:
Released in 1979, the SG-1 essentially filters out the attack of a signal and swells in the volume from your guitar. It was discontinued in 1982. However, this type of BOSS effect and technology would go on to live into the future with other BOSS multi-effect releases. The SG-1 was fairly innovative for its time and created a sound similar to a bowed instrument making the guitar sound more like a cello or violin.
Rarity: Rare
Usefulness: Moderate -High
Price: $230 - $500
Cheaper Alternative: Mooer Slow Engine
Example: 
Vibrato VB-2
Based on the modulation circuit from the JC-120 Roland Jazz Chorus amp. Discontinued in less than a year into production makes this pedal incredibly rare. Created a warble type of tone very familiar in a lot of music today. In 2016 an update “WAZAcraft” BOSS version of this pedal was released with an updated and modded circuit.
Rarity: Very Rare
Usefulness: Moderate
Price: $200 - $375
Cheaper Alternative: Behringer UV300 Ultimate Vibrato
Example: 