I'll second the L3, I have one for use with my Total Bithead. You can get most of Fiio's LODs for under $10 on Amazon. Avoid the cheapest ones they make (there's one or two cheaper-made models, as well as knock-offs.)
Also, think about whether you want a short cable like the L3 or one of the longer-cable options - many amps will pick up interference from the phone, so you might have a better experience using a longer cable. I've found it's only a real issue when my phone is using EDGE, but sometimes you can't control that (and your mileage can, and will, vary based on cell coverage and the amp you use.) Nowadays I seem to consistently get 3G, which admittedly can still cause minor, occasional noise, but it's ignorable (and easy to drown out) and I can avoid most of it simply by not putting the phone directly on top of the amp. If I were still stuck having to put up with EDGE a lot, I would've opted for an LOD with a longer cable.
If you don't have the amp yet, I'd say you can't go wrong with the L3 (especially for the low price) but it might not be a bad idea to get the amp first and see how it behaves with a plain mini-to-mini cable before buying an LOD. Even a fairly generic cable with an amp will most likely be an improvement over the stock headphone jack by itself in the meantime.
