MASTER TOOLKIT by OCP for the 16K/48K Spectrum is a collection of 27 routines to make programing easier. The routines include a clock, with alarm, which can be displaying the time continually, even when back in Basic, and routines to delete, renumber lines, find, list or substitute a given string for another string or variable and display a list of variables. You can delete all REM statements, lower RAMTOP and print the amount of free memory.
The useful routines are Trace, where each line number is displayed as it is executed and ON ERROR GO TO and BREAK, where when the program stops running and tries to print a report code, i.e., when BREAK is pressed, a jump is made to a specific line.
The Pack routine squeezes as much as possible into multi-statement lines, taking note of IFs, REMs and GO TOs.
You are also given 10 programmable function keys, which can be used from Basic. It is very comprehensive and worth buying if you do much programming.