The Government Inspector at The Young Vic, near Waterloo.
Straight off the bat, I highly recommend it. We didn't really know what to expect of the seats, just £10 for a stool in the gallery. But, they afford an excellent aerial view, you can lean on the balcony bar if you get tired of sitting up straight, and the stools are well cushioned. (Hooray!)
The play itself is a comedy about corruption, paranoia and mistaken identity, set in a small provincial town in Russia. The town mayor hears word that a government official is coming to inspect the town, throwing (the corrupt) local officials into panic and confusion.
Meanwhile, Khelstakov, a foppish young layabout, dressed in the latest St Petersburg fashions, is stranded in the town's inn, having lost all his money in a card game.
He is then (unsurprisingly) mistaken for the Government Inspector, with hilarious consequences...
The best known faces in this are of course Julian Barratt (of The Mighty Boosh) as the Mayor and Doon Mackichan (of Smack The Pony) as the Mayor's wife, who are both marvellous, but there are excellent performances throughout the cast.
Kyle Soller is excellent as Khlestakov, and also standing out were Louise Brealey as the Mayor's vain daughter, Amanda Lawrence as the nosey Postmaster, and whoever plays the creepy German doctor (I can't find a proper cast list by character, and Google Search results are inconclusive on this one).
More about the history of the play on Wikipedia.