Macoscope has released version 2.0 of its Xcode navigation plug-in, Code Pilot. Technically, it's still a beta -- judging from the 2.0b6 version number -- but it's been rock solid for me so far. This version brings Xcode 4 compatibility and a revamped interface. If you code and you haven't tried it, definitely take a look.
Code Pilot attaches to Shift-Command-X in Xcode, and the hotkey brings up a HUD with a search field focused. As you type, it quickly and intelligently filters the files in your project down to the one you were looking for. It also searches symbol names across the board or within the selected file (add a space after the filename). It's similar to Xcode's Open Quickly, but smarter and easier to install than PeepCode's PeepOpen (which is great in MacVim and TextMate but a bit less friendly in Xcode).
Macoscope, also behind Productive! Magazine for iPad and the Nozbe apps for iPad and iPhone, has gained a reputation for refined design. Code Pilot is a great example of smart, unobtrusive UI with some serious utility behind it. The free download is good for 10 days, at which point you'll need a license (US$29.95). Users who purchased v1.x in 2011 get v2.0 for free, and 1.x owners who purchased before that get a 50% discount.