GameSpy 3D (www.gamespy.com) is an excellent utility that makes navigating the mess of servers out there much easier. Here's a couple tips on using GameSpy that make it easier to find 'Oz' modified servers for Half-Life.

Set up an Oz filter: if you're an Oz-only player (or Counterstrike, or other mod), you can set up what's called a SmartSpy filter to display only servers of that type. The filtering is done on their servers now, so it's much faster than in previous versions. Here's how. First, right-click on the Half-Life suitcase and select 'Add SmartSpy List':

Fill out the Properties sheet. The following example is for Oz:

Now, simply right-mouse that new entry and do 'Update':

The servers listed will be Oz servers. (Note: certain admins have tricked their servers into thinking they're Valve servers, even though they're Oz (TheLowerLevel servers), so they don't show up. However, the SmartSpy list picks up the vast majority. I've manually added TheLowerLevel servers to my Unbound list.