grep gives you the biggest bang for the buck. You can use it to search on any type code and many forms of data. It is fast, and very powerful. In code it can locate what you're looking for in variables and function names, but also in comments. You can also pipe results into it, and can thus enhance the utility of many tools available on site.
With some clever hints you can easily make grep search for a specific type of an identifier. For instance, "^function_name" will often find in C code a function's definition, because these start with the name of the function at the beginning of the line. If a search pattern gives you too many false matches, you can filter those out, by piping the result through grep -v.
Many years ago I was stranded debugging COBOL programs on a 1970s-era Perkin Elmer machine running OS/32. The machine lacked programming tools, but had an ancient C compiler (so old, it would accept =+ as the original form of the += operator). I ended up writing a rudimentary grep program, which immensely improved my productivity.
