TextMate is one of my favorite companions these days. It's the pinnacle text editing, but unfortunately, only for Mac OS X.
What makes it so great? A few things:
- Rock hard stable. I've never had it crash. It's responsive.
- Ridiculously configurable, yet so simple. There are tons and tons of plugins and bundles out there do practically whatever you want, to support nearly every language and platform. If there is something you want that isn't out there, you can write it.
- Directory editing. You can open a whole directory in a sidebar, easily getting at any files. This is enormously useful for most web frameworks, where you have to spider through a hierarchy to get at the various components.
- Generous licensing. The normal single user license allows you to install it on every computer that you use, including even you work computer. (As long as it is meant for your use.)
With so much done right (and trust me, there's even more), what could possibly be so wrong about it? Well, two things. First, it's OS X only (though there are some lookalikes available for other platforms). Second, it's a tad pricey, sitting at roughly $59 right now.
(P.S. I've even started to ditch my trusty old TeXShop in favor of it, due to TeXShop's poorer integrated PDF viewer and extreme memory leaks. (TextMate's TeX mode launches it in a full Preview window.))
(P.P.S. Anyone doing serious TeX work should be using OS X anyways: it's the only platform that has built-in grammar checking, so you can finally grammar check your TeX!)