Windows/Mac + iPhone/iPod touch: Not only do videos take up a ton of space on space-constrained devices, but converting videos for the iPhone gets painful quickly. Air Video streams videos straight to your iPhone, converting them on-the-fly if they're incompatible.
The app is actually an iPhone app plus a PC/Mac app that acts as a server. You can use any videos on your computer, whether they are in iTunes or not—and, if you prefer, you can also add video playlists from iTunes to the list of sources. Once you get the server app running on your desktop, you can start streaming over your home network immediately. Streaming over the internet from outside your home, though, only takes a few more seconds—in the server app, go to the "Remote" tab and check Enable Access from Internet (see below). The app will give you a server PIN that you can type in when you go to add a source on your iPhone or iPod touch—note that your router at home needs a public IP address and support UPnP or NAT-PMP protocols, which shouldn't be a problem for most people.
As if that weren't cool (or easy) enough, if you have some videos that can't be played directly on the iPhone, you can convert them using Air Video as you watch it (as long as you're running firmware 3.0 and have a fairly powerful computer back at home). If you prefer, you can also convert the file offline and watch it later.
Air Video is a free download for the iPhone and iPod touch, although the free version only shows you a few videos at a time, at random, from your folders. If you have a large video library you want to share, or don't feel like clicking on the folder multiple times waiting for the video you want to be on the list, there's a $2.99 pro version available as well.