When it comes to birth control, it's a lot harder figuring out how to stop a bajillion little sperm than one little egg, but a scientist in India has managed to develop an injection for men that has so far proven to be 100% effective and completely reversible with no side effects. Sign me up.
The injection is called RISUG, which stands for "reversible inhibition of sperm under guidance." Let's just get the uncomfortable bit out of the way first: you can't just be injected anywhere, the doctor has to stick a needle into your scrotum. Twice. Yeah, bad times, but you'll deal, and here's why: after two tiny injections taking all of 15 minutes, study after study has shown that the injection is 100% effective at preventing pregnancy. You won't suffer any side effects at all, it lasts a minimum of 10 years, and it's completely reversible with another few injections.
Here's how it works: a non-toxic, positively charged polymer gets injected into the little tube that your sperm pass through on their way from your testicles to, you know, out. The polymer doesn't actually block the tube, but it stays there, and when sperm (which have a negative charge) pass by, the charge differential from the polymer zaps them. All of them. Every time.
So why is this not available yet? Well, not to go all conspiracy theory on you, but drug companies aren't especially interested in RISUG because they wouldn't make any money on it. The injection lasts a decade and the syringe would likely end up costing more than what's inside, so unlike male hormonal birth control (which they've been working on for years), patients wouldn't need to keep paying for more medication every month.
Luckily, there's a private foundation that's working to get RISUG approved here in the US, although they have to start more or less from scratch to do it. The company is called Parsemus, and they recently got $100k from the Gates Foundation to see if RISUG might work on women, too.
In the meantime, Phase III trials are starting up in India, which is basically the last step before RISUG gets approved for use by anyone who wants it (in India, anyway) within the next few years.