If you are looking for a low cost NAS you surely will come across with three devices: the Linksys NSLU2, the Iomega MyBook WE and the D-Link DNS-323 (or its sibling CH3SNAS pitcured on left).
Devices with similar functionality (all of them support Samba/FTP/ITunesServer/UPNPAVServer/etc…) and that in a similar fashion have been greatly extended thanks to the community. The NSLU2 with its custom firmware, linux distributions and the optware package repository, the MyBook WE with a first customized firmware and now with a full fledged community and the DNS-323/CH3SNAS with the fonz’s fun_plug and the huge amount of modifications that came after that.
Something similar to what always happens to devices based on FOSS and when the opportunity to enhance them is given to the community and something,  in my opinion, that always boosts the “saleability” of such products, just think to the famous Linksys WRT54G.
I got my own Conceptronic CH3SNAS last year, and since then i enjoyed the progress made by the community and contributed my little share when possible.
You can now find all my DNS323/CH3SNAS binaries here. Feel free to try them and call for support if needed.

After this long preamble about opensource and NASes, let’s talk briefly about Linux and embedded development.
There are many ways to start learning Linux for those who come from a MSWindows background such as installing a random distribution, maybe on a virtual machine, and so on…
But what the DNS323 and similar devices give you is the opportunity to get a taste of what embedded development is.
The hardware platform reminds what you could find in your average pocket-pc and is good enough for this use:  a Marvell SoC with an ARM926Tj at 500Mhz and 64Mb of RAM.
Don’t limit your experience to installing a different UPnP AV Server or the latest Transmission/mldonkey, try to compile some software yourself and experiment.
Browse the WIKI to discover what a toolchain is, how you can cross-compile software for different CPUs (in this case an ARM926) on a x86 host and which are the main differences among glibc and libraries like uclibc, eglibc, etc…
This post should contain enough pointers for you to start .

Open-source electronics platforms for rapid prototyping have already been around for a while, i can think of Arduino board but surely there are more of them.
The low cost, the wide variety of inputs/outputs, the ability to interact with sensors ot to control motors, the ease of use and the community, make these platforms a perfect fit for hobbyist’s and artist’s projects.
Regarding the ease of use, Arduino can be programmed in a c-like language ( additional straightforward libraries to access the board’s functions are provided ) and has a development environment based on the famous Processing.
And now, SUN Spot.

I’ve spotted the video embedded below on the googletechtalks youtube channel.

The platform developed by SUN Laboratories seems awesome:

Completely open source (hardware/software)

180 MHz 32 bit ARM920T core (with jazelle?) - 512K RAM/4M Flash

2.4 GHz IEEE 802.15.4 radio with integrated antenna

USB interface (host?)

2G/6G 3-axis accelerometer

Temperature sensor

Light sensor

8 tri-color LEDs

6 analog inputs

2 momentary switches

5 general purpose I/O pins and 4 high current output pins

100 mt max distance between Spots (more info on this on the sunspotworld.com forum, the devices together form a meshed network, so multiple stations can be used to extend the network civerage, but the maximum distance between two spots seems limited to that)

Missing agps and only 4mb flash you say? Looks like someone already built a custom gps module and SD module for their projects. Some new addons have been shown at JavaOne2008.

The SUN Spot can be programmed enterely in Java, and has a development platform based on NetBeans (Eclipse can also be used).
Refer to the embedded video for some example projects. Above all, watch @00:21 for the cool Sun Blackbox monitoring project.
It comes in a developer kit with 1 Spot base station and 2 standard Spots for under $700, a kit that reminds me of the early openmoko developers kit.
The two things that i found most intriguing are the ability to upload new “software packages” wirelessly (in this presentation is done through a management interface, could this be done dinamically using the java api provided?) and the way the meshed wireless network formed by Spots is managed (i should take a look at this sooner or later).
Additional videos about Sun Spot and projects based on it can be found on youtube.