I use Tiny Hexer by mirkes.de - just Google that and you'll find plenty of download sites, though http://www.mirkes.de
doesn't look like the right place to go - it is not being actively supported there. The main reason I like TH is that it have a Structure Viewer (I think that is what it is called) that converts the hex code into sensible floating-point numbers on the fly. So, using this tool, you can just type in the new decimal value you want it to be, without having to find some conversion tool first. I don't know what features Neo has going for it, so I can't say for certain that TH is better, but I really like it.
The characters that show on the right panel are just the ASCII text equivalents for those bytes, and I guess there are just several bytes which have no ASCII character equivalent. They all get rendered the same way. One thing that really tripped me up at first was that "00" and "20" looked the same - "00" tells RRT3 that it has reached the end of a text string, whereas "20" is the space-bar character, still a part of the text string. If I goofed between those two, then it would get the name wrong and fail to load assets. Another thing you'll notice is that the original programmers of RRT3 were rather sloppy with their text strings... they would just type the new name, up to the "00", then leave the rest of the junk beyond that point. So you end up with a bunch of nonsense in the text string portions, which doesn't affect how the game functions, but can be confusing to you as you mod.