Way to improve "show me this decimal number to M places, don't use scientific notation"?

You can express any fraction/number to arbitrary decimal places by using a backtick followed by number of digits required. For example:

In[1]:= 4/3`20
Out[1]= 1.3333333333333333333

This is the same as N[4/3, 20]. Now combine this with AccountingForm, which never uses scientific notation to get the output that you desire.

AccountingForm[1/998001`2994]
Out[2]//AccountingForm= 0.0000010020030040050060070080090100110120130140...

However, be aware that AccountingForm uses parentheses for negative numbers:

AccountingForm[-1/998001`2994]
Out[3]//AccountingForm= (0.00000100200300400500600700800901001101201301401501601....

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Daniel Lichtblau has a good point that although using `instead of N might be shorter in this case, in general, it might not give the same result — for example, compare the digits of Log[2`50] and N[Log[2],50]. You'll see that they differ in the last couple of digits. However, for small use cases, the difference might be insignificant.

You could always set $Post to have this happen automatically.

format[x_Real] := NumberForm[x, ExponentFunction -> (Null &)];
format[x_] := x;
$Post = format;

Now,

N[1/998001, 50]

returns

0.0000010020030040050060070080090100110120130140150160170

Even better, $Post is applied at display time, thus

Head[%]

returns Real.