eTokki is highly recommended (ps2->360; no passthrough)
Another one is the Xconverter 360 (eTokki used to carry this but replaced it because it requires a 360 PCB passthrough).
If you can get the SuperUFO Controller Converter Pro (http://superufo.com/product_html/Xbox_1___360_xBox360_Controller_converter_PRO.html) these are also great (can do PS2 or PS3! (controller) to 360 without a 360 PCB passthrough.
There's also this one: http://superufo.com/product_html/Xbox_1___360_PS2_Controller_to_XBOX_360_Converter.html that does PS2 -> 360 (requires 360 pcb passthrough)
Those are your main options.
Your CHEAPEST option is probably this one: http://www.dealextreme.com/p/ps2-to-xbox-360-controller-adapter-cable-33241 that you can find at various online retailers/eBay.
It's iffy though. That last option hasn't been thoroughly tested and although they're (for the most part) lagless, they get a little glitchy. Sometimes they hold inputs (directions) sometimes they drop/add inputs (it seems). Most people have had very little problems with them, but you're taking a gamble when you buy them. For the most part, anecdotal SRK/Youtube reviews praise this converter but like I said, no extensive testing has been done.
Another one is the Xconverter 360 (eTokki used to carry this but replaced it because it requires a 360 PCB passthrough).
If you can get the SuperUFO Controller Converter Pro (http://superufo.com/product_html/Xbox_1___360_xBox360_Controller_converter_PRO.html) these are also great (can do PS2 or PS3! (controller) to 360 without a 360 PCB passthrough.
There's also this one: http://superufo.com/product_html/Xbox_1___360_PS2_Controller_to_XBOX_360_Converter.html that does PS2 -> 360 (requires 360 pcb passthrough)
Those are your main options.
Your CHEAPEST option is probably this one: http://www.dealextreme.com/p/ps2-to-xbox-360-controller-adapter-cable-33241 that you can find at various online retailers/eBay.
It's iffy though. That last option hasn't been thoroughly tested and although they're (for the most part) lagless, they get a little glitchy. Sometimes they hold inputs (directions) sometimes they drop/add inputs (it seems). Most people have had very little problems with them, but you're taking a gamble when you buy them. For the most part, anecdotal SRK/Youtube reviews praise this converter but like I said, no extensive testing has been done.