From this position my opponent played:

c1=B+

Needless to say I won the game.

I don't mind when people play out lost positions, but if you aren't playing to at least draw do the courteous thing and resign!

Why does it matter what he promoted to? Your king was going to get it anyways. Lighten up, it's not like it's against the rules or even considered bad sportsmanship.

I see nothing wrong with someone playing c1=B+ especially if it's just a casual game. I'm sure if he or she is a serious tournament player playing in a serious game, then he or she would have acted differently. If you're playing for fun, then have fun; he or she was following the rules :)

there's nothing wrong with underpromotion just for the randomness of it.  It's unnecessary, sure, but there's no harm done.

It's kind of funny if you think about it.

In an ironic sense, it is sort of funny.

i underpromote if my opponent doesn't resign when clearly lost. nothing as humilliating as being checkmated by 3 or 4 knights! :)

I do it occasionally in that kind of situation to see if the opponent will let me leave the piece on, i.e. not immediately take it like they would a queen.  Its purely an attempt by myself to confuse them and cause a momentary lapse in concentration which might give me an advantage in an otherwise lost position for me.  Needless to say, it hasn't worked yet!

Wouldn't it be OK to promote to a Knight if, say that move would checkmate your opponent's King? I see nothing wrong with it. That is a position that a lesser rated player might no think of.

Also it keeps it challenging

Underpromotion to anything but a knight is meaningless from the game standpoint (Q does both the moves of R and B) - except in cases where a Q will lead to a stalemate. However, the player may play it for psychological reasons, as mentioned by Erik and headofwords.

The best way to deal with a prank is to take it sportingly and retaliate in like fashion.

white has a few mates on the board without underpromoting, so this is a bad case, but you are right.  White can play Qd8# and Bf6# as well as c8=N#.  Below is a better example.

 I never thought of it like that, dsrkar. You are absolutely right. After promotion, with a queen you can run both ranks-files and diagonals. But aren't there some situations where promoting to a queen would set up a stalemate? So to avoid this, a Rook or Bishop would be better?

Underpromotion is almost never a prank.  It is an invaluable tactic.  Here is another example of its great worth.

 Yep! That's what I said earlier.

I'm extremely proud of the game where in a game my opponent refused to resign or even capture pieces that I had left hanging, so I decided to promote all my pawns to knights, and checkmated with 7 knights. 

 Nice sentiment. Instead of winning as quickly as possible against an opponent who won't resign in a dead lost position. I like to start my King on a long stroll across the board. I have only been at this site a short time but my opponents have all been very decent and shown excellent chess etiquette.

One time, my opponent was being an idiot, so I promoted to three dark squared bishops and a queen.  He was taunting me the entire game.

In direwolfe's position, I don't think c1=B+ is taunting at all.  It would have been taunting if you had let him keep the bishop!

just be careful because taunting can sometimes bite you hard. Some guy decided to be clever and used 2 knights and 2 bishops, and I got stalemate