matrix mixer, sequential switch, kinks, logic and quite frankly all the other ones, it really depends on you
have a look at a few different types and try and work out how you would use them - particularly for modulation purposes (sa you mention that) - extending what you already have in more interesting ways than 'just getting another eg/lfo' by mixing, modifying, multing and switching
my other suggestion would be to spend the money on the next case - then you really won't care about the fact you only have 14 hp left
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities