Everyone knows that you can snap InDesign frames to guides, by simply dragging an object close to a guide. (Of course, View > Grids & Guides > Snap to Guides must be selected for this to work). But did you know that you can also snap guides to frames? You can, and this is really handy sometimes. Here's the trick:
First, select the frame you want to snap a guide to. Next, drag a guide near the selected frame edge, but then make sure that your cursor actually touches one of the selection handles on the frame. When you do this, you will feel the guide "snap" to the edge of the frame. If the "snap" is too subtle for you to feel, choose Preferences > Guides & Pasteboard and change the Snap to Zone to a larger value. This will increase the "power" of the magnetic attraction between guides and objects.
New in CS4: The new "Smart Guides" in InDesign CS4 (improved in Illustrator CS4) will make many guide-based tasks obsolete. The more I work with Smart Guides, the more amazed I am by how well they work. This is a feature that you'll need to see to appreciate, but it allows you to almost effortlessly align the edges of frames without using Guides or the Align panel.