Depending on what you need to use a controller adapter, there are steps that need to be taken to decide. The most obvious beging system type, What controller do you want to use on your PC or Mac? A Nintendo 64 controller? An Xbox dance pad? 4 Simultaneous playstation 2 controllers?

1. - N64 controllers - There is only one adapter in this case, so your choice is easy. N64 USB Adapter - buy now! 
The upside, is that this will turn your N64 controller into a PC compatible controller for any emulation software or other game software that supports controllers. The downside is that some random N64 controllers don't work very well with this adapter, but since this is the only adapter made, you should give it a try. Multiple adapters CAN be used on the same computer for multiplayer

2. - Dance pads/Stepmania - These NEED to be able to support multiple key presses at the same time. Most come with a dance pad "mode" that you need to enable by holding certain buttons to activate it, make sure to check your retail packaging! Click here for all dance pad compatible adapters  Some of these support up to four controllers/dance pads at once! Multiple adapters CAN be used on the same computer for multiplayer

3. - Playstation 1 and 2 adapters. While the N64 adapter supports playstation controllers, it's definitely not the best or most cost effective way to go about it, try one of these instead depending on how many controllers you want to use at the same time - Playstation 1 and 2 USB Adapters

4. - PS2 controllers on other systems like gamecube and xbox - 3 types of PS2 to alternate system adapters  The one to go for here is without a doubt the 3 in 1 adapter. This lets you use dance pads on other systems, and turns your PS2 controller into a controller for the PC, Gamecube, AND xbox. If you want to save some money and only need it for one system, get either the xbox or gamecube adapter.

5. - SNES Controller adapter.- SNES USB Controller adapters  Again, only one choice, but this is also one of the best controller adapters ever. Fully compatible with any emulator or game program that supports controllers, this is an EXCELLENT choice, some auctions also include a brand new SNES controller! Multiple adapters CAN be used on the same computer for multiplayer

With next gen systems already coming out, the prices on these may go down, but so will the availability and options. Now is the time to buy one!