My blog, Iron Rogue, was mostly about triathlons. I was, however, always interested in other ways multiple disciplines could be combined into a single endurance event. This list is by no means exhaustive, but it's reasonably comprehensive if you ask me.
Swim, Bike, Run. These vary from "Try-A-Tri" all the way to the famous "Iron Man" distance.
If you take out the swim from a triathlon and Run-Bike-Run, you have a duathlon. Some people don't want to swim, and sometimes Triathlons cancel their swims due to weather/water conditions.
3. Swim Run
I haven't tried these, but there's no bike, and you pretty much switch from swimming across a body of water to running to swimming several times. You take your shoes with you.
4. Off-Road Triathlon
If the running is on trails, and the bikes are mountain bikes, this is the event.
A famous winter sport. Cross-country skiing is a demanding endurance sport, and right when your blood is really pumping, you have to be calm and still to target shoot with a rifle.
6. Winter Triathlon
Run in the snow, mountain bike in the snow, then cross-country ski.
7. Archer's Paradox
Like a summer biathlon - trail running and target archery in one event.
8. Adventure Race
Adventure Racing is an outdoor endurance sport for teams including a combination of mountain biking, trail running/trekking, paddle sports (kayak/canoe/raft), and map and compass navigation.
9. Pentathlon Des Neiges
Another unique event in Quebec City. Bike, run, cross-country ski, ice skate (!) and snowshoe.
10. Ski to Sea
I've had my eye on this one for years (there used to be a Canadian event, but this one is close to me, so maybe one day soon). It's a team relay and takes some organization as you descend from the mountains to the sea - cross country ski <-to-> downhill ski <-to-> run<-to-> road bike <-to-> canoe <-to-> cyclocross bike <-to-> sea kayak. Cyclocross could be considered a multisport event in and of itself, since it often involves dismounting from the bike and running with it (sometimes over obstacles).