Non-Linux FOSS: All the Bitcoin, None of the Bloat
I love Bitcoin. Ever since I first discovered it in 2010 and mined thousands of them, I've been hooked on the technology, the concept and even the software. (Sadly, I sold most of those thousands of Bitcoin when they were less than a dollar. I'm still kicking myself.) One of the frustrations with using Bitcoin, however, is that the blockchain has gotten so large. It currently weighs in at a little less than 20GB, and it takes about a week to download the first time you do so. There are ways to jumpstart the download with a bootstrap file, but still, it's a huge undertaking to run the standard Bitcoin client.
Although it doesn't have the entire blockchain, MultiBit does have all of the security and encryption the standard Bitcoin client does. Because it reads the Bitcoin network in real time, it takes seconds to sync up as opposed to days. Sending and receiving Bitcoin with MultiBit is fast, efficient and secure. Oh, and it's open-source, multiplatform and under constant development! If you want to run a local Bitcoin client, but don't want to download the entire blockchain, check out MultiBit at http://multibit.org. (Also check out Electrum, a Python-based alternative at http://electrum.org.)