winmerge is an open source diff/merge tool with a really good UI. comparable linux apps are meld and kdiff3, but winmerge is more capable than meld and feels a lot more polished than kdiff3. I'm actually surprised no one has ported it to linux, though I presume a lot of the polish is due to focusing on look and feel in a way that is tied to the underlying windows gui libraries.

proxomitron is a rewriting proxy, which I always thought was a very nice approach to webpage filtering. again, I remember it having very good UI/UX as well as being fast and capable.