Transmit 4's new Disk feature allows you to mount an FTP favorite as a volume in the Finder. That means any application can have native-ish FTP support with both download and upload capability (unlike the one-way FTP support built into the Finder).
This is a simple hint, and an extension of the Transmit 4 documentation. But since it's directly extending the Finder's functionality independent of the Transmit App itself, I thought it hint-worthy. I'm finding this especially useful with Aperture, which doesn't have built-in FTP support, although you can purchase a rather expensive plug-in for FTP.
After mounting the destination favorite via the menubar Transmit icon, the volume will show up in the Finder sidebar, and also in any Open/Save dialog. In Aperture, this also allows access to the volume through the Export menu item, which additionally allows resizing, renaming, and foldering before the export is completed. This will save many a step in the uploading of photos to remote servers, and brings us one step closer to true OS-level FTP support.
[robg adds: If you just want system-level FTP support, and not all of Transmit 4's useful features, you can install MacFUSE -- this is exactly what Transmit 4 does, in fact. We've run a number of hints on MacFUSE, so if you want to dive in, there are hints that can help.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100427103944103