yes, most people use HAMMER.  i say most because i'm not sure if there is another program that can be used.  HAMMER is free and to get it you must download the SDK package from steam (this is also free).  the package comes with a lot of stuff you won't need, but since HAMMER runs off of SDK, you have to get it and start it everytime you want to use HAMMER.

as for decompiling a map, there are many programs you can use.  i don't know which one i use because i'm at work, but your best bet is to just type "decompile .bsp to .vsp" into google and you will probably get a bunch of results.  of course these programs are free too.  if you see one that you need to pay for, just skip it for there are enough good free ones out there that paying for one isn't necessary.