My Accent has generally been a good enough car, but recently (at the 65k mile mark) it's started losing about half a quart of oil between changes (which are generally about every 3500 miles). The problem is much worse around town than it is on long-distance trips (by which I mean houston-los angeles or houston-new york). There's a lot of knocking and tapping going on under my hood, especially when idling, but there has been since the car was brand new so I don't know how to distinguish any of it from anything else. Also, there's some oil visible around the engine from time to time, but not a lot. Gaskets? Seals? Anyone got any ideas?

If your hearing knocking get your timing belt checked.If that breaks pray that your warranty covers it big$$

Loosing that much oil is essentially normal. If you were driving a saturn, you'd find it closer to two quarts. The knocking is something to be concerned about. Timing belt as suggested is a good place to start. also a possibility your EGR valve is stuck open causing a spark knock. Either way, I'd get it looked at soon...

The oil around the engine is probably a bad valve gasket. Not too expensive, labor will be a bit if you get it done at a shop.