Create a formal mentorship program. Build it into the HR Title System. Senior Developers mentor Junior, etc. Formalize the goals for the program with HR and build it into their job description.
Give homework. “We’re doing a lot of Threading work lately, here’s some assigned reading.”
Inspire a culture of learning. If you’re (assuming you’re the Alpha Geek) not giving off an encouraging and enthusiastic vibe, your developers have little reason to be excited themselves.
Create a Thanks A Bunch Cabinet. Corillian had a great system where peers could reward peers. Our administrator, Kate, had a budget given her by the Engineering Department. I think it was less than $1000 a quarter. She’d go and get as much cool stuff as she could, each under $25. This included iTunes cards, stuff from Sharper Image, Flashlights, just nerdy stuff. If you went above and beyond in some aspect of your job, one of your peers could reward you by sending you to the TABCab. You get to go home with a cool, unexpected free gift, and your peers get to tell you you’re appreciated. It was a great system.