I found myself in exactly the same spot. I've worked on a PHP web application for the past four years, and eventually realized my experience was very one-dimensional. However, I feel like I've gone through some explosive growth the past couple months, and am now very active in pursuing learning. Here are some of the things I've found:
The more you can expose yourself to others' code, the better. Seek out programming blogs that actually teach you something (example: 10 Things I Learned from the jQuery source). Follow or participate in an open-source project. Look over the shoulder of other developers on your team.
Make a hobby of programming. Find a project you can spend a couple hours on each weekend that you enjoy, and stretch yourself. This is a safe environment to experiment with new approaches and make mistakes in a low-stress atmosphere. We learn a lot from mistakes, but I've found that at work, when a deadline is looming, I rarely take the time to stop what I'm doing and research a better solution to a problem. At home, there are no deadlines, so you can look stuff up all you want.
Teach yourself a new language or framework. In my case, I decided to learn Python and am now building a home finance web app in Pylons. StackOverflow is invaluable at speeding up learning time, too; ask questions!
Browse Stack Overflow, and when you stumble across something unfamiliar, research it. And believe it or not, answering questions will help you learn, too. Answering forces you to clearly think through what you know (or think you know) and articulate it in a precise way. I've found this is especially true on topics I have just recently learned about.