Komodo Edit – the open source spin-off of Komodo IDE – is hands down my personal open source application of the year. It had a massive impact on my productivity and it made web development so much less of a pain.
I used so many different text editors in my life, but none of them comes anywhere near the sheer awesomeness of Komodo. There is proper Unicode support, smart indent (which actually behaves smart), remote file editing (FTP, FTPS, SFTP, and SCP), Code Intelligence (~IntelliSense™), and lots of polish on top. It supports many languages and runs on the three most popular platforms (i.e. Windows, Mac OS, and Linux). It's also extremely stable, which is very important for this kind of tools.
The ActiveState guys are also very helpful and surprisingly responsive. For example there were some issues with active mode FTP, but after a bit of nagging and poking around in the code (mostly nagging though) the problem was solved. Just like that. I also asked for some shortcut which basically does the same as a double click; selecting the word below the caret. A few minutes later the JavaScript macro was there (thanks Todd!) and it even made it's way upstream; the next version of Komodo will include it as a regular command.