The people over at CrunchBang Linux have put together what they promise is a lightweight Ubuntu-based distribution. It comes in Standard, Lite and Eee versions. I have yet to try it, but I absolutely love the minimalist default desktop and am very impressed by what I've read.
CrunchBang's approach to implementation and design differs a bit from what I'm trying to do (ever so slowly lately) with SkinnyDebbie. Implementation-wise, CrunchBang is a full distribution that is based on a standard distribution (Ubuntu); SkinnyDebbie is a configuration that sits on top of a standard (currently Debian) distribution. In terms of design, without actually trying it I can't say for sure, but it looks as though CrunchBang is aimed at users that have at least a bit of Linux familiarity and a tiny bit of hardware headroom.; the design philosophy behind SkinnyDebbie is to make it as useful and inviting as possible to computer newbies while using the lightest-weight core options that still get the job done.
Of course neither approach is better than the other; CrunchBang and Skinny are intended for slightly different audiences/flavors of users. If you are looking for a lightweight Linux distribution, please give CrunchBank a spin and let me know what you think. If CrunchBang had been around when I started Skinny, I might never have started it. :-)