YourITronics

The circuit uses a constant current drive and a four wire connection to the probes. This allows almost any length of probe connecting cable without re-calibration. Surface mount chips are wonderful, but for prototype work these can be difficult to use. If you only have SMD parts available to make it easier you can construct a SO to DIP adapter.

Two Channel Thermometer: [Link]

This is actually version 2 of the syringe logic probe, so it has some improvements over the first one. The advantages of using a syringe as a logic probe are obvious, you can reach small pins or solder points. The probe wassn’t yet tested at high frequencies, but it works well enough for sniffing AES keys from an SPI bus.

Syringe Logic Probe: [Link]