1. headphone amplifier - CMOY type, small, great for using good headphones with portable music players. uses a good audio OP Amp (like Analog Devices OP275).

2. unbalanced to balanced audio transmitter and balanced to unbalanced audio receiver - great for transmitting audio signals over great distances while eliminated any noise picked up on the wires. The circuit inverts the audio signal 180 degrees and transmits the in-phase and out-of-phase signals on a pair of wires, the receiver inverts the out-of-phase signal providing 3dB of gain and eliminating any picked up noise by 70dB or more.

3. video switched 120V outlet - input is the output video signal from a tv, when a video signal is present, the circuit energizes a relay to give power to an amplifier or powered speaker system. with this the audio system will turn on/off with the tv.

4. an alternatve to #3 is a circuit that senses an audio signal instead - common auto turn-on circuit in modern sub amplifiers.

5. solar panel battery charger - intelligent battery charging circuit

6. HDTV antenna booster amplifier

7. UHF/VHF channel modulator - accepts any video signal over RG59/RG6 coaxial cable and modulates it to a cable channel in either the UHF or VHF band, while allowing standard cable operation. example - security camera at the front door can be viewed from any TV in the house on channel 54. If the product can allow for bi-directional signals and HDTV though Cable you'd have a product that you could market.