Just a Timex Ironman digital watch. It tells time, has a stopwatch, and never runs out of battery. No GPS because a good watch is expensive and I already spend enough on shoes. Works fine for me.
Another vote for the Timex Ironman
I had the 50 or 100 lap version in college. I have the 30 lap version now, 100 lap function would be handy for some longer workouts (for example 20 x 400m with 200m jogging rest). I didn't know how good I had it back then.
You can get by with 20 x 4 with the 30 lap version. Just don't record the rests. That's probably about the most reps you'll do in a workout ever though. The only time I thought more than 30 laps might be useful was in recording mile splits for an ultra.