I can thoroughly recommend Under Armour BoxerJock undercrackers.
I had been running in either Nike or Karrimor 2 in 1 shorts with built in undershorts. Mainly because I've never gotten on with those with mesh liners.
Having recently bought a couple of pairs of Aldi running shorts in their recent running event, I realised I needed something to wear underneath. A bit of a poke on the interweb bought the Under Armour BoxerJocks (6 inch style) to my attention. I ordered a couple of pairs and did my first run in them this morning.
They were absolutely fantastic, really comfortable, kept everything in place. They are about £10 a pair but I'm thinking of gradually replacing my standard M&S boxers with them.