Wartime production Fabrique Nationale Pistole "GP" ("Grand Puissance"), other wise known as the Browning Hi-Power. A 13-shot, 9mm Parabellum, single-action, semi-auto. The first modern high capacity military semi-auto to make a big splash in the world. When the Germans overran Belgium early in WWII, they simply appropriated the superlative design for issue to their own forces and had FN continue production. The early occupation weapons maintained the highly polished blue finish for which the weapon was famous, but as the war progressed, weapons produced for the Germans received a more utilitarian "military" finish (like yours). Guns built for business...not looks.
The Hi-Power is a truly classic handgun, in continuous production since before WWII, and still highly admired today. Once upon a time, it was the pistol which equipped many of our NATO allies and many other countries around the world. It was especially popular in Great Britain, where the weapon was used by most units, including the SAS (through the early 90s IIRC). Actually, some UK military outfits continue to issue them to this day. One of my UK Liaison Officers was packing a Browning just a year or two ago while in Iraq.
Go here for more info:
http://www.hipowersandhandguns.com/index.html
Be advised that the author of the site, Stephen Camp, passed away some months ago. His website is THE definitive source for info on BHPs.
General Description:
http://en.wikipedia.org/wiki/Browning_Hi-Power
Value guestimate for your pistol (German issued, tangent sight, late war production, in the condition depicted): ~ $950. Possibly well over a thousand if you are patient and run it at auction on Gunbroker.com.
Although the weapon displays rust & patina, ONLY CLEAN LIGHTLY IF YOU PLAN TO SELL. Your weapon appears to be all original, and collectors prefer original honest finish. If you intend to keep it, a little light bronze cleaning brush work with plenty of gun oil will take away some of the rust, but don't try to "polish the gun". You will detract from its sales value.
Here's one on GunBroker with asking price of $1075 (and an aftermarket barrel):
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=251475845#PIC
Realistically, another ~$950 gun. Or less.