Here are the contact details (year 2007) of some consulates and high commissions that are located in Karachi:
2-a/1, St 20, Phase 5, Defence Society.
12/A, Mohammad Ali Bogra Road, Bath Island.
Tel: +92-21-5873797 / 5873798.
92-A/7, Block 5, Clifton.
Tel-A: +92- 21-5873782 / 83.
British Deputy High Commission:
Shahrah-e-Iran, Clifton 75600.
Fondest memory: INDIA:
India House 3, Fatima Jinnah Road,
Post Box No. 8542, Karachi.
Tel: +92-21-522275 / 512542.
Organon Pakistan Bldg; 36-A/1 Lalazar Near M.T. Khan Road.
(opp. Beach Luxury Hotel) Karachi.
Tel: +92-21-5611618 / 1628.
Clifton No 98, Karachi 75600, Pakistan.
Tel: +92-21-5873987 / 990.
UNITED STATES OF AMERICA:
Abdullah Haroon Road, Karachi - Pakistan.
If you need to visit the Chinese Embassy to get a VISA in Karachi, be sure to not be intimidated by the crowd outside, or bother waiting in line (or you will never get in). The Embassy is open 9-12 Mon, Wed, Friday, and takes 2 days to process.
Just walk up to the door, knock, and when it opens flash your (non-Pakistani) passport to get in. It's rude, but unfortunately necessary
There is also a Chinese Embassy in Islamabad.