I'm not sure if their website is good or not, but K.K Rabbit has their little book of deliveries. They go to the restaurant, pick it up, and deliver it for you, even if the restaurant itself doesn't deliver.

You can find that book all over Beijing, it's orange and has K.K. Rabbit written in big.

It's the best I know so far.

The booklet Mr_Spoon mentions above is definitely the most comprehensive guide to restaurants that deliver and I've referred to it many times. You might also get more info about restaurants on localnoodles.com. I think they do mainly reviews, but it's worth checking out.

I did a google search and also found a website called Jinshisong.com. You can choose from Chinese or Western restaurants and they deliver. I've never used it myself so I can't guarantee that is good or even legit, but it may also be worth looking into now that the freezing winter days are looming closeby.

jinshisong.com

I found this website a few days ago, and they deliver for a ton of good restaurants. 

Blue Frog, La Sushi, Sukhothai, Kros Nest etc.

Delivery fees are a bit expensive though if you don't live close to central Chaoyang.