I'm no expert but here's my take. You want to be looking for 406 MHz PLBs (Personal Locator Beacons), ELTs (Emergency Locator Transmitters) and/or EPIRBs (Emergency Position-Indicating Radio Beacons) that use the Cospas-Sarsat international satellite system.
Generally:
EPIRBs = maritime distress
ELTs = aircraft distress
PLBs = personal use
FYI: Spot doesn't use the 406 MHz signal or the same satellites. As you may already know SPOT uses GlobalStar satelites. The only one I've heard of that's anything like Spot is called inReach.
The 406 PLB range includes: ACR Electronics, McMurdo FastFind & Emerging Lifesaving Technologies.
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SPOT Review -
SPOT 2 GPS Messenger: "Lifeline, Peace of MInd" by Nutnfancy
SPOT vs PLB
The 406 devices have a 'stronger', more reliable & reassuring signal but the unite are more expensive. However, they don't have expensive annual subscriptions like SPOT. Conversely, the 406 units do not have the useful tracking and 'I'm OK' functions. The 406 units really are a last resort device. In conclusion I think it depends where you're going, who, if anyone is worrying about you, and what the terrain is likely to be.
