The problem with pretty much all USB Centronics (LPT) adapters that you can find on-line is that they only support higher-level printer communications.
For a non-printer device such as an OPLxLPT, a Covox Speech Thing or a CNC interface to work, the parallel port interface must support bit banging, which basically means that it should be able to rapidly switch the bits on and off at each of the I/O pins, with accurate timing.
Although USB 2.x and 3.x should be able to offer the required low latency for this (when operating in the right mode), no such fully-featured USB parallel port adapters currently exist in the wild, as far as I know.
A direct USB OPLx solution like the one matze79 linked to above would therefore be more practical, I think.
(But it would still be cool if someone finally developed a fully-featured USB parallel port adapter that wouldn't just work with printers.)