Micomsoft's X-RGB Series
Micomsoft's X-RGB series of upscan convertors (aka VGA boxes) is the most capable and functional series in the world. If there's anything better, I don't know what it is. Upscan convertors convert signals from TV-friendly 15kHz to 31kHz, allowing low-res video game consoles to be displayed on VGA monitors.
The main reason for the Micomsoft's usefulness and popularity is their ability to upscan RGB signals, which no other consumer-level convertor does. By starting with RGB you're getting the best signal quality possible, better than component, better than S-video and much better than composite video.
This was the first in their line of adaptors, and while perfectly functional it lacks some of the finesse of the later units. Adjustments are accomplished through DIP switches, and adjustability is limited. It accepts Composite or S-video, and RGB.
This is the best device for gamers looking for an all-purpose solution. It supports composite video, S-video and RGB. The biggest improvement over the X-RGB1 is the inclusion of onscreen menus, gamma adjustment, and artificial scan-lines, re-creating the effect of low-res monitors.
The X-RGB2+ is not as useful as its predecessors, but more useful in some respects. It uses a soft-switching passthrough which introduces some blur into the VGA signal when it's not in use, but this was the price paid for a remote control. It offers more output resolutions, but doesn't support arcade PCBs any longer. It does offer additional inputs, including component video via a Japanese D-terminal connector (and the 21-pin RGB connector doubles as a second component input). It's less of a gamer's all-around solution and more of a home-theatre device for those willing to compromise a little.
It is more like the 2+ than the 2, offering many more output resolutions, additional inputs, but still does not support arcade gaming. It also offers output via DVI-D, making it possible to use a DVI-D to HDMI cable, but has lag in this mode due to full processing. Another mode is available (B1 mode, not available in stock units but simple to install via official means) which is quite similar to an X-RGB2+.