Stripe for invoicing (Update: invested in PAID, stripe ended up also doing invoice)

Better business banking (Update: invested in SEED, Mercury)

Cheap android robots

Ultra cheap MRI scanners 

Modulatable/writable nanopore for DNA printing

Ornicopter for drones

Quantum dot sterling engine

Makerbot for biology

Low cost brain scanners

A better google glass / magic leap (direct into eye raster scan)

Super low latency / high bandwidth cell tower networks

Electric supersonic planes [Invested in BOOM, invested in Wright electric :>]

Electric commercial planes (Update: invested in Wright Electric)

Venture debt marketplace

Better Email / Bullet proof better than gmail email [Invested Superhuman / Polymail]

p2p bitcoin lending [ended up happening]

Unjammable laser shredder

Metamaterial engines

Ultrasonic non-contact tactile holodeck

Ornicopters for submarines

Ornicopter disc helicopter

Surface tension MEMS devices

Knowledge engine 

Backpack gyro for space dive flat spin

Particle accelerators on chip

Ultrasonic cavitation for hydrofoils

Greencoin - environmentally friendly bitcoin / computationally less intensive mechanics [ended up happening]

Blockchain and synthetic biology [Nebula genomics]

Quantum Geneva Drive (see quantum/soft gears)

Lytro of sound for a microphone

Vortex pulse detonation engine 

Negative skin drag metamaterials

Atomic level JTEC or on a carbon nanotube

Super cavitating hydrofoils for surfcraft

Laser superposition processor

Quantum gears

Memresitor / memcap / meminductor analogy to thermal / magnetic components via circuit theory. 

Metamaterials applied to aerofoils for greater lift

Plasma surface control for supersonic props.

Bullet proof MEMS/nuclear battery based aircraft instrumentation

Flux self trapping engine (e.g. zero wear space reaction wheels)

Auto CRISPR machine

Fractal surface cookware for maximum heat transfer

Nuclear PDE/PJE

Superconducting shaftless turbo pulse jet/detonation engine

Pulse resonant rocket

Hybrid turbofan rocket with energy recovery from the nozzle

Retractable landing wings for reusable rockets 

P2P Wonga (lower rates)

Dynamic plasma wing (reading the uncoming airpressure)

Lightweight radar based crevasse detector for mountaineering

Visual version of Shepard tone

Rocket thrusters for auto flat spin recovery

Metamaterial array for reducing blast waves

Feel free to ping me if you built any of these.