Sunomomo. Typically prepared with Japanese cucumber, but it works fine with other types too. Recipe:

• 4 cucumbers or so
• ½ Tbsp salt
• 1 Tbsp sugar
• 2 Tbsp water
• 3 Tbsp rice vinegar or white wine vinegar
• 1 tsp roasted sesame seeds

1. Slice the cucumbers, really thin. It's fine to use a knife, but do it preferably with a mandolino.
2. Sprinkle salt over the slices. Make sure that it's spread evenly. Wait 15min or so. Drain the water.
3. In the meantime mix the sugar, water, and vinegar. Add it to the cucumber and sprinkle the sesame seeds. Let it chill a bit and serve.

Some people also add soy sauce, or MSG.