Top 3 cheap rollerball refills for your better pens

Below are three low cost refill alternatives for rollerballs that fit a surprising number of higher-end rollerball pens, and often providing a superior writing experience to the original refill. For most, the refill should fit without alteration, but sometimes due to a difference in refill length, shape or step down, it may require a small alteration (eg. remove an end cap, trim the refill length or add a spacer such as a small wad of paper or eraser.) See the notes at the end for troubleshooting tips. These refills are all low cost, so it doesn't cost much to try. If you prefer not to experiment, check if Monteverde supplies a aftermarket compatible refill that has been specifically designed to fit your pen.

Rollerballs accepting Pentel Energel refills:
Faber-Castell Essentio, Hexo, Neo
Lamy 2000 (instead of M63) with a spacer such as trimmed plug off M63
Lamy Safari/Al-Star (instead of M63) with a spring or spacer 
Papermate InkJoy Gel
Pentel Energel
Pilot Dr Grip, G2 Edge, G2 Limited, Juice
Retro 51 Tornado
Sailor Tuzu
Sharpie S-Gel
Uni One, Signo 207
Zebra Sarasa

Rollerballs accepting Pilot G2 refills:
(if harvesting from a Pilot pen use the spring as well)
Faber-Castell Essentio, Hexo, Neo
Kaweco
Lamy 2000 (instead of M63) with a spacer such as trimmed plug off M63
Lamy Safari/Al-Star (instead of M63) with a spring or spacer 
Lamy Swift with refill end cap removed
Pentel Energel compatible pens (see below)
Pilot B2B, Dr Grip, G2 Edge, G2 Limited, Juice, MR
Retro 51 Tornado
Rotring 600
Sheaffer
Tactile Turn
Waterman

Rollerballs accepting Uniball Signo 207 refills:
Lamy Safari (instead of M63) with a spring or spacer
Lamy Tipo (instead of M66)
Pentel Energel
Pilot G2 Limited
Rotring 600
Tactile Turn
Uniball

Troubleshooting tips:
If the substitute refill is loose at the writing end, try increasing the diametre of the front end of the refill with a piece of tape applied smoothly. If the substitute refill is a touch short, try replacing the spring from a different pen. Small adjustments to how firmly the nose cone is screwed in may also help if the clicker is not quite engaging with the refill.