Fast growing function Aside from the power, gamma, exponential functions are there any other very fast growing functions in (semi-) regular use?

Knuth's uparrow notation grows pretty fast.  Basically, it goes as follows:
$$a\uparrow b=a^b$$
$$a\uparrow\uparrow b=a^{a^{a^{\dots}}}\bigg\}b\text{ amount of }a's$$
$$a\uparrow\uparrow\uparrow b=\underbrace{a\uparrow\uparrow(a\uparrow\uparrow (\dots(a\uparrow\uparrow a)\dots))}_{b\text{ amount of }a's}$$
etc. you get the main idea.  It's use?  It's a good way to represent large numbers, the most famous of which is known as Graham's number, which is the upper bound to a problem in Ramsey theory:

Possibly just as famous as the above, we have the Ackermann function.  It's original usage was to show that not all total computable functions are primitive recursive, and it eventually built its way into things like Computability theory and computer science.
Within the realm of googology, one of the arguably most useful function is the fast growing hierarchy.  It is useful because its simple and it can easily be used to rank most functions.  For example, it is not only able to rank against addition, multiplication, and exponentiation, but it can easily extend further, providing non-trivial decent lower bounds to even the TREE sequence:
$$\text{TREE}(n)\ge f_{\vartheta(\Omega^\omega\omega)}(n)$$
