Reddit is already quite light compared to Digg, but a young coder named Peteris Krumins has created an ever lighter version. Reddit River is "3 times as light (in size)" and it has the benefit of automatically discovering mobile versions of stories posted on Reddit, which effectively saves you some precious MBs. What usually happens in these situations is that Reddit creates their own mobile version in a matter of weeks, but I applaud Peteris for his efforts.
I've tried it out on my trusty Sony Ericsson P1i, and it works as advertised - it's really fast, and I was surprised at how good it was with regard to discovering mobile versions of stories. Have in mind that this project has been hacked together in a small amount of time, so don't expect tons of features.